perldebug - Online in the Cloud

PROGRAM:

NAME

perldebug - Perl debugging

DESCRIPTION

First of all, have you tried using the -w switch?

If you're new to the Perl debugger, you may prefer to read perldebtut, which is a tutorial
introduction to the debugger.

The Perl Debugger

If you invoke Perl with the -d switch, your script runs under the Perl source debugger.
This works like an interactive Perl environment, prompting for debugger commands that let
you examine source code, set breakpoints, get stack backtraces, change the values of
variables, etc. This is so convenient that you often fire up the debugger all by itself
just to test out Perl constructs interactively to see what they do. For example:

$ perl -d -e 42

In Perl, the debugger is not a separate program the way it usually is in the typical
compiled environment. Instead, the -d flag tells the compiler to insert source
information into the parse trees it's about to hand off to the interpreter. That means
your code must first compile correctly for the debugger to work on it. Then when the
interpreter starts up, it preloads a special Perl library file containing the debugger.

The program will halt right before the first run-time executable statement (but see below
regarding compile-time statements) and ask you to enter a debugger command. Contrary to
popular expectations, whenever the debugger halts and shows you a line of code, it always
displays the line it's about to execute, rather than the one it has just executed.

Any command not recognized by the debugger is directly executed ("eval"'d) as Perl code in
the current package. (The debugger uses the DB package for keeping its own state
information.)

Note that the said "eval" is bound by an implicit scope. As a result any newly introduced
lexical variable or any modified capture buffer content is lost after the eval. The
debugger is a nice environment to learn Perl, but if you interactively experiment using
material which should be in the same scope, stuff it in one line.

For any text entered at the debugger prompt, leading and trailing whitespace is first
stripped before further processing. If a debugger command coincides with some function in
your own program, merely precede the function with something that doesn't look like a
debugger command, such as a leading ";" or perhaps a "+", or by wrapping it with
parentheses or braces.

Calling the Debugger
There are several ways to call the debugger:

perl -d program_name
On the given program identified by "program_name".

perl -d -e 0
Interactively supply an arbitrary "expression" using "-e".

perl -d:ptkdb program_name
Debug a given program via the "Devel::ptkdb" GUI.

perl -dt threaded_program_name
Debug a given program using threads (experimental).

Debugger Commands
The interactive debugger understands the following commands:

h Prints out a summary help message

h [command] Prints out a help message for the given debugger command.

h h The special argument of "h h" produces the entire help page, which is quite
long.

If the output of the "h h" command (or any command, for that matter) scrolls
past your screen, precede the command with a leading pipe symbol so that it's
run through your pager, as in

DB> |h h

You may change the pager which is used via "o pager=..." command.

p expr Same as "print {$DB::OUT} expr" in the current package. In particular,
because this is just Perl's own "print" function, this means that nested data
structures and objects are not dumped, unlike with the "x" command.

The "DB::OUT" filehandle is opened to /dev/tty, regardless of where STDOUT may
be redirected to.

x [maxdepth] expr
Evaluates its expression in list context and dumps out the result in a pretty-
printed fashion. Nested data structures are printed out recursively, unlike
the real "print" function in Perl. When dumping hashes, you'll probably
prefer 'x \%h' rather than 'x %h'. See Dumpvalue if you'd like to do this
yourself.

The output format is governed by multiple options described under
"Configurable Options".

If the "maxdepth" is included, it must be a numeral N; the value is dumped
only N levels deep, as if the "dumpDepth" option had been temporarily set to
N.

V [pkg [vars]]
Display all (or some) variables in package (defaulting to "main") using a data
pretty-printer (hashes show their keys and values so you see what's what,
control characters are made printable, etc.). Make sure you don't put the
type specifier (like "$") there, just the symbol names, like this:

V DB filename line

Use "~pattern" and "!pattern" for positive and negative regexes.

This is similar to calling the "x" command on each applicable var.

X [vars] Same as "V currentpackage [vars]".

y [level [vars]]
Display all (or some) lexical variables (mnemonic: "mY" variables) in the
current scope or level scopes higher. You can limit the variables that you
see with vars which works exactly as it does for the "V" and "X" commands.
Requires the "PadWalker" module version 0.08 or higher; will warn if this
isn't installed. Output is pretty-printed in the same style as for "V" and
the format is controlled by the same options.

T Produce a stack backtrace. See below for details on its output.

s [expr] Single step. Executes until the beginning of another statement, descending
into subroutine calls. If an expression is supplied that includes function
calls, it too will be single-stepped.

n [expr] Next. Executes over subroutine calls, until the beginning of the next
statement. If an expression is supplied that includes function calls, those
functions will be executed with stops before each statement.

r Continue until the return from the current subroutine. Dump the return value
if the "PrintRet" option is set (default).

<CR> Repeat last "n" or "s" command.

c [line|sub]
Continue, optionally inserting a one-time-only breakpoint at the specified
line or subroutine.

l List next window of lines.

l min+incr List "incr+1" lines starting at "min".

l min-max List lines "min" through "max". "l -" is synonymous to "-".

l line List a single line.

l subname List first window of lines from subroutine. subname may be a variable that
contains a code reference.

- List previous window of lines.

v [line] View a few lines of code around the current line.

. Return the internal debugger pointer to the line last executed, and print out
that line.

f filename Switch to viewing a different file or "eval" statement. If filename is not a
full pathname found in the values of %INC, it is considered a regex.

"eval"ed strings (when accessible) are considered to be filenames: "f (eval
7)" and "f eval 7\b" access the body of the 7th "eval"ed string (in the order
of execution). The bodies of the currently executed "eval" and of "eval"ed
strings that define subroutines are saved and thus accessible.

/pattern/ Search forwards for pattern (a Perl regex); final / is optional. The search
is case-insensitive by default.

?pattern? Search backwards for pattern; final ? is optional. The search is case-
insensitive by default.

L [abw] List (default all) actions, breakpoints and watch expressions

S [[!]regex]
List subroutine names [not] matching the regex.

t [n] Toggle trace mode (see also the "AutoTrace" option). Optional argument is the
maximum number of levels to trace below the current one; anything deeper than
that will be silent.

t [n] expr Trace through execution of "expr". Optional first argument is the maximum
number of levels to trace below the current one; anything deeper than that
will be silent. See "Frame Listing Output Examples" in perldebguts for
examples.

b Sets breakpoint on current line

b [line] [condition]
Set a breakpoint before the given line. If a condition is specified, it's
evaluated each time the statement is reached: a breakpoint is taken only if
the condition is true. Breakpoints may only be set on lines that begin an
executable statement. Conditions don't use "if":

b 237 $x > 30
b 237 ++$count237 < 11
b 33 /pattern/i

If the line number is ".", sets a breakpoint on the current line:

b . $n > 100

b [file]:[line] [condition]
Set a breakpoint before the given line in a (possibly different) file. If a
condition is specified, it's evaluated each time the statement is reached: a
breakpoint is taken only if the condition is true. Breakpoints may only be
set on lines that begin an executable statement. Conditions don't use "if":

b lib/MyModule.pm:237 $x > 30
b /usr/lib/perl5/site_perl/CGI.pm:100 ++$count100 < 11

b subname [condition]
Set a breakpoint before the first line of the named subroutine. subname may
be a variable containing a code reference (in this case condition is not
supported).

b postpone subname [condition]
Set a breakpoint at first line of subroutine after it is compiled.

b load filename
Set a breakpoint before the first executed line of the filename, which should
be a full pathname found amongst the %INC values.

b compile subname
Sets a breakpoint before the first statement executed after the specified
subroutine is compiled.

B line Delete a breakpoint from the specified line.

B * Delete all installed breakpoints.

disable [file]:[line]
Disable the breakpoint so it won't stop the execution of the program.
Breakpoints are enabled by default and can be re-enabled using the "enable"
command.

disable [line]
Disable the breakpoint so it won't stop the execution of the program.
Breakpoints are enabled by default and can be re-enabled using the "enable"
command.

This is done for a breakpoint in the current file.

enable [file]:[line]
Enable the breakpoint so it will stop the execution of the program.

enable [line]
Enable the breakpoint so it will stop the execution of the program.

This is done for a breakpoint in the current file.

a [line] command
Set an action to be done before the line is executed. If line is omitted, set
an action on the line about to be executed. The sequence of steps taken by
the debugger is

1. check for a breakpoint at this line
2. print the line if necessary (tracing)
3. do any actions associated with that line
4. prompt user if at a breakpoint or in single-step
5. evaluate line

For example, this will print out $foo every time line 53 is passed:

a 53 print "DB FOUND $foo\n"

A line Delete an action from the specified line.

A * Delete all installed actions.

w expr Add a global watch-expression. Whenever a watched global changes the debugger
will stop and display the old and new values.

W expr Delete watch-expression

W * Delete all watch-expressions.

o Display all options.

o booloption ...
Set each listed Boolean option to the value 1.

o anyoption? ...
Print out the value of one or more options.

o option=value ...
Set the value of one or more options. If the value has internal whitespace,
it should be quoted. For example, you could set "o pager="less -MQeicsNfr""
to call less with those specific options. You may use either single or double
quotes, but if you do, you must escape any embedded instances of same sort of
quote you began with, as well as any escaping any escapes that immediately
precede that quote but which are not meant to escape the quote itself. In
other words, you follow single-quoting rules irrespective of the quote; eg: "o
option='this isn\'t bad'" or "o option="She said, \"Isn't it?\""".

For historical reasons, the "=value" is optional, but defaults to 1 only where
it is safe to do so--that is, mostly for Boolean options. It is always better
to assign a specific value using "=". The "option" can be abbreviated, but
for clarity probably should not be. Several options can be set together. See
"Configurable Options" for a list of these.

< ? List out all pre-prompt Perl command actions.

< [ command ]
Set an action (Perl command) to happen before every debugger prompt. A multi-
line command may be entered by backslashing the newlines.

< * Delete all pre-prompt Perl command actions.

<< command Add an action (Perl command) to happen before every debugger prompt. A multi-
line command may be entered by backwhacking the newlines.

> ? List out post-prompt Perl command actions.

> command Set an action (Perl command) to happen after the prompt when you've just given
a command to return to executing the script. A multi-line command may be
entered by backslashing the newlines (we bet you couldn't have guessed this by
now).

> * Delete all post-prompt Perl command actions.

>> command Adds an action (Perl command) to happen after the prompt when you've just
given a command to return to executing the script. A multi-line command may
be entered by backslashing the newlines.

{ ? List out pre-prompt debugger commands.

{ [ command ]
Set an action (debugger command) to happen before every debugger prompt. A
multi-line command may be entered in the customary fashion.

Because this command is in some senses new, a warning is issued if you appear
to have accidentally entered a block instead. If that's what you mean to do,
write it as with ";{ ... }" or even "do { ... }".

{ * Delete all pre-prompt debugger commands.

{{ command Add an action (debugger command) to happen before every debugger prompt. A
multi-line command may be entered, if you can guess how: see above.

! number Redo a previous command (defaults to the previous command).

! -number Redo number'th previous command.

! pattern Redo last command that started with pattern. See "o recallCommand", too.

!! cmd Run cmd in a subprocess (reads from DB::IN, writes to DB::OUT) See "o
shellBang", also. Note that the user's current shell (well, their $ENV{SHELL}
variable) will be used, which can interfere with proper interpretation of exit
status or signal and coredump information.

source file Read and execute debugger commands from file. file may itself contain
"source" commands.

H -number Display last n commands. Only commands longer than one character are listed.
If number is omitted, list them all.

q or ^D Quit. ("quit" doesn't work for this, unless you've made an alias) This is the
only supported way to exit the debugger, though typing "exit" twice might
work.

Set the "inhibit_exit" option to 0 if you want to be able to step off the end
the script. You may also need to set $finished to 0 if you want to step
through global destruction.

R Restart the debugger by "exec()"ing a new session. We try to maintain your
history across this, but internal settings and command-line options may be
lost.

The following setting are currently preserved: history, breakpoints, actions,
debugger options, and the Perl command-line options -w, -I, and -e.

|dbcmd Run the debugger command, piping DB::OUT into your current pager.

||dbcmd Same as "|dbcmd" but DB::OUT is temporarily "select"ed as well.

= [alias value]
Define a command alias, like

= quit q

or list current aliases.

command Execute command as a Perl statement. A trailing semicolon will be supplied.
If the Perl statement would otherwise be confused for a Perl debugger, use a
leading semicolon, too.

m expr List which methods may be called on the result of the evaluated expression.
The expression may evaluated to a reference to a blessed object, or to a
package name.

M Display all loaded modules and their versions.

man [manpage]
Despite its name, this calls your system's default documentation viewer on the
given page, or on the viewer itself if manpage is omitted. If that viewer is
man, the current "Config" information is used to invoke man using the proper
MANPATH or -M manpath option. Failed lookups of the form "XXX" that match
known manpages of the form perlXXX will be retried. This lets you type "man
debug" or "man op" from the debugger.

On systems traditionally bereft of a usable man command, the debugger invokes
perldoc. Occasionally this determination is incorrect due to recalcitrant
vendors or rather more felicitously, to enterprising users. If you fall into
either category, just manually set the $DB::doccmd variable to whatever viewer
to view the Perl documentation on your system. This may be set in an rc file,
or through direct assignment. We're still waiting for a working example of
something along the lines of:

$DB::doccmd = 'netscape -remote http://something.here/';

Configurable Options
The debugger has numerous options settable using the "o" command, either interactively or
from the environment or an rc file. (./.perldb or ~/.perldb under Unix.)

"recallCommand", "ShellBang"
The characters used to recall a command or spawn a shell. By default, both
are set to "!", which is unfortunate.

"pager" Program to use for output of pager-piped commands (those beginning with a "|"
character.) By default, $ENV{PAGER} will be used. Because the debugger uses
your current terminal characteristics for bold and underlining, if the chosen
pager does not pass escape sequences through unchanged, the output of some
debugger commands will not be readable when sent through the pager.

"tkRunning" Run Tk while prompting (with ReadLine).

"signalLevel", "warnLevel", "dieLevel"
Level of verbosity. By default, the debugger leaves your exceptions and
warnings alone, because altering them can break correctly running programs.
It will attempt to print a message when uncaught INT, BUS, or SEGV signals
arrive. (But see the mention of signals in "BUGS" below.)

To disable this default safe mode, set these values to something higher than
0. At a level of 1, you get backtraces upon receiving any kind of warning
(this is often annoying) or exception (this is often valuable).
Unfortunately, the debugger cannot discern fatal exceptions from non-fatal
ones. If "dieLevel" is even 1, then your non-fatal exceptions are also traced
and unceremoniously altered if they came from "eval'ed" strings or from any
kind of "eval" within modules you're attempting to load. If "dieLevel" is 2,
the debugger doesn't care where they came from: It usurps your exception
handler and prints out a trace, then modifies all exceptions with its own
embellishments. This may perhaps be useful for some tracing purposes, but
tends to hopelessly destroy any program that takes its exception handling
seriously.

"AutoTrace" Trace mode (similar to "t" command, but can be put into "PERLDB_OPTS").

"LineInfo" File or pipe to print line number info to. If it is a pipe (say,
"|visual_perl_db"), then a short message is used. This is the mechanism used
to interact with a slave editor or visual debugger, such as the special "vi"
or "emacs" hooks, or the "ddd" graphical debugger.

"inhibit_exit"
If 0, allows stepping off the end of the script.

"PrintRet" Print return value after "r" command if set (default).

"ornaments" Affects screen appearance of the command line (see Term::ReadLine). There is
currently no way to disable these, which can render some output illegible on
some displays, or with some pagers. This is considered a bug.

"frame" Affects the printing of messages upon entry and exit from subroutines. If
"frame & 2" is false, messages are printed on entry only. (Printing on exit
might be useful if interspersed with other messages.)

If "frame & 4", arguments to functions are printed, plus context and caller
info. If "frame & 8", overloaded "stringify" and "tie"d "FETCH" is enabled on
the printed arguments. If "frame & 16", the return value from the subroutine
is printed.

The length at which the argument list is truncated is governed by the next
option:

"maxTraceLen"
Length to truncate the argument list when the "frame" option's bit 4 is set.

"windowSize"
Change the size of code list window (default is 10 lines).

The following options affect what happens with "V", "X", and "x" commands:

"arrayDepth", "hashDepth"
Print only first N elements ('' for all).

"dumpDepth" Limit recursion depth to N levels when dumping structures. Negative values
are interpreted as infinity. Default: infinity.

"compactDump", "veryCompact"
Change the style of array and hash output. If "compactDump", short array may
be printed on one line.

"globPrint" Whether to print contents of globs.

"DumpDBFiles"
Dump arrays holding debugged files.

"DumpPackages"
Dump symbol tables of packages.

"DumpReused"
Dump contents of "reused" addresses.

"quote", "HighBit", "undefPrint"
Change the style of string dump. The default value for "quote" is "auto"; one
can enable double-quotish or single-quotish format by setting it to """ or
"'", respectively. By default, characters with their high bit set are printed
verbatim.

"UsageOnly" Rudimentary per-package memory usage dump. Calculates total size of strings
found in variables in the package. This does not include lexicals in a
module's file scope, or lost in closures.

"HistFile" The path of the file from which the history (assuming a usable Term::ReadLine
backend) will be read on the debugger's startup, and to which it will be saved
on shutdown (for persistence across sessions). Similar in concept to Bash's
".bash_history" file.

"HistSize" The count of the saved lines in the history (assuming "HistFile" above).

After the rc file is read, the debugger reads the $ENV{PERLDB_OPTS} environment variable
and parses this as the remainder of a "O ..." line as one might enter at the debugger
prompt. You may place the initialization options "TTY", "noTTY", "ReadLine", and
"NonStop" there.

If your rc file contains:

parse_options("NonStop=1 LineInfo=db.out AutoTrace");

then your script will run without human intervention, putting trace information into the
file db.out. (If you interrupt it, you'd better reset "LineInfo" to /dev/tty if you
expect to see anything.)

"TTY" The TTY to use for debugging I/O.

"noTTY" If set, the debugger goes into "NonStop" mode and will not connect to a TTY.
If interrupted (or if control goes to the debugger via explicit setting of
$DB::signal or $DB::single from the Perl script), it connects to a TTY
specified in the "TTY" option at startup, or to a tty found at runtime using
the "Term::Rendezvous" module of your choice.

This module should implement a method named "new" that returns an object with
two methods: "IN" and "OUT". These should return filehandles to use for
debugging input and output correspondingly. The "new" method should inspect
an argument containing the value of $ENV{PERLDB_NOTTY} at startup, or
"$ENV{HOME}/.perldbtty$$" otherwise. This file is not inspected for proper
ownership, so security hazards are theoretically possible.

"ReadLine" If false, readline support in the debugger is disabled in order to debug
applications that themselves use ReadLine.

"NonStop" If set, the debugger goes into non-interactive mode until interrupted, or
programmatically by setting $DB::signal or $DB::single.

Here's an example of using the $ENV{PERLDB_OPTS} variable:

$ PERLDB_OPTS="NonStop frame=2" perl -d myprogram

That will run the script myprogram without human intervention, printing out the call tree
with entry and exit points. Note that "NonStop=1 frame=2" is equivalent to "N f=2", and
that originally, options could be uniquely abbreviated by the first letter (modulo the
"Dump*" options). It is nevertheless recommended that you always spell them out in full
for legibility and future compatibility.

Other examples include

$ PERLDB_OPTS="NonStop LineInfo=listing frame=2" perl -d myprogram

which runs script non-interactively, printing info on each entry into a subroutine and
each executed line into the file named listing. (If you interrupt it, you would better
reset "LineInfo" to something "interactive"!)

Other examples include (using standard shell syntax to show environment variable
settings):

$ ( PERLDB_OPTS="NonStop frame=1 AutoTrace LineInfo=tperl.out"
perl -d myprogram )

which may be useful for debugging a program that uses "Term::ReadLine" itself. Do not
forget to detach your shell from the TTY in the window that corresponds to /dev/ttyXX,
say, by issuing a command like

$ sleep 1000000

See "Debugger Internals" in perldebguts for details.

Debugger Input/Output
Prompt The debugger prompt is something like

DB<8>

or even

DB<<17>>

where that number is the command number, and which you'd use to access with the
built-in csh-like history mechanism. For example, "!17" would repeat command
number 17. The depth of the angle brackets indicates the nesting depth of the
debugger. You could get more than one set of brackets, for example, if you'd
already at a breakpoint and then printed the result of a function call that itself
has a breakpoint, or you step into an expression via "s/n/t expression" command.

Multiline commands
If you want to enter a multi-line command, such as a subroutine definition with
several statements or a format, escape the newline that would normally end the
debugger command with a backslash. Here's an example:

DB<1> for (1..4) {
cont: print "ok\n";
cont: }
ok
ok
ok
ok

Note that this business of escaping a newline is specific to interactive commands
typed into the debugger.

Stack backtrace
Here's an example of what a stack backtrace via "T" command might look like:

$ = main::infested called from file 'Ambulation.pm' line 10
@ = Ambulation::legs(1, 2, 3, 4) called from file 'camel_flea' line 7
$ = main::pests('bactrian', 4) called from file 'camel_flea' line 4

The left-hand character up there indicates the context in which the function was
called, with "$" and "@" meaning scalar or list contexts respectively, and "."
meaning void context (which is actually a sort of scalar context). The display
above says that you were in the function "main::infested" when you ran the stack
dump, and that it was called in scalar context from line 10 of the file
Ambulation.pm, but without any arguments at all, meaning it was called as
&infested. The next stack frame shows that the function "Ambulation::legs" was
called in list context from the camel_flea file with four arguments. The last
stack frame shows that "main::pests" was called in scalar context, also from
camel_flea, but from line 4.

If you execute the "T" command from inside an active "use" statement, the
backtrace will contain both a "require" frame and an "eval" frame.

Line Listing Format
This shows the sorts of output the "l" command can produce:

DB<<13>> l
101: @i{@i} = ();
102:b @isa{@i,$pack} = ()
103 if(exists $i{$prevpack} || exists $isa{$pack});
104 }
105
106 next
107==> if(exists $isa{$pack});
108
109:a if ($extra-- > 0) {
110: %isa = ($pack,1);

Breakable lines are marked with ":". Lines with breakpoints are marked by "b" and
those with actions by "a". The line that's about to be executed is marked by
"==>".

Please be aware that code in debugger listings may not look the same as your
original source code. Line directives and external source filters can alter the
code before Perl sees it, causing code to move from its original positions or take
on entirely different forms.

Frame listing
When the "frame" option is set, the debugger would print entered (and optionally
exited) subroutines in different styles. See perldebguts for incredibly long
examples of these.

Debugging Compile-Time Statements
If you have compile-time executable statements (such as code within BEGIN, UNITCHECK and
CHECK blocks or "use" statements), these will not be stopped by debugger, although
"require"s and INIT blocks will, and compile-time statements can be traced with the
"AutoTrace" option set in "PERLDB_OPTS"). From your own Perl code, however, you can
transfer control back to the debugger using the following statement, which is harmless if
the debugger is not running:

$DB::single = 1;

If you set $DB::single to 2, it's equivalent to having just typed the "n" command, whereas
a value of 1 means the "s" command. The $DB::trace variable should be set to 1 to
simulate having typed the "t" command.

Another way to debug compile-time code is to start the debugger, set a breakpoint on the
load of some module:

DB<7> b load f:/perllib/lib/Carp.pm
Will stop on load of 'f:/perllib/lib/Carp.pm'.

and then restart the debugger using the "R" command (if possible). One can use "b compile
subname" for the same purpose.

Debugger Customization
The debugger probably contains enough configuration hooks that you won't ever have to
modify it yourself. You may change the behaviour of the debugger from within the debugger
using its "o" command, from the command line via the "PERLDB_OPTS" environment variable,
and from customization files.

You can do some customization by setting up a .perldb file, which contains initialization
code. For instance, you could make aliases like these (the last one is one people expect
to be there):

$DB::alias{'len'} = 's/^len(.*)/p length($1)/';
$DB::alias{'stop'} = 's/^stop (at|in)/b/';
$DB::alias{'ps'} = 's/^ps\b/p scalar /';
$DB::alias{'quit'} = 's/^quit(\s*)/exit/';

You can change options from .perldb by using calls like this one;

parse_options("NonStop=1 LineInfo=db.out AutoTrace=1 frame=2");

The code is executed in the package "DB". Note that .perldb is processed before
processing "PERLDB_OPTS". If .perldb defines the subroutine "afterinit", that function is
called after debugger initialization ends. .perldb may be contained in the current
directory, or in the home directory. Because this file is sourced in by Perl and may
contain arbitrary commands, for security reasons, it must be owned by the superuser or the
current user, and writable by no one but its owner.

You can mock TTY input to debugger by adding arbitrary commands to @DB::typeahead. For
example, your .perldb file might contain:

sub afterinit { push @DB::typeahead, "b 4", "b 6"; }

Which would attempt to set breakpoints on lines 4 and 6 immediately after debugger
initialization. Note that @DB::typeahead is not a supported interface and is subject to
change in future releases.

If you want to modify the debugger, copy perl5db.pl from the Perl library to another name
and hack it to your heart's content. You'll then want to set your "PERL5DB" environment
variable to say something like this:

BEGIN { require "myperl5db.pl" }

As a last resort, you could also use "PERL5DB" to customize the debugger by directly
setting internal variables or calling debugger functions.

Note that any variables and functions that are not documented in this document (or in
perldebguts) are considered for internal use only, and as such are subject to change
without notice.

Readline Support / History in the Debugger
As shipped, the only command-line history supplied is a simplistic one that checks for
leading exclamation points. However, if you install the Term::ReadKey and Term::ReadLine
modules from CPAN (such as Term::ReadLine::Gnu, Term::ReadLine::Perl, ...) you will have
full editing capabilities much like those GNU readline(3) provides. Look for these in the
modules/by-module/Term directory on CPAN. These do not support normal vi command-line
editing, however.

A rudimentary command-line completion is also available, including lexical variables in
the current scope if the "PadWalker" module is installed.

Without Readline support you may see the symbols "^[[A", "^[[C", "^[[B", "^[[D"", "^H",
... when using the arrow keys and/or the backspace key.

Editor Support for Debugging
If you have the GNU's version of emacs installed on your system, it can interact with the
Perl debugger to provide an integrated software development environment reminiscent of its
interactions with C debuggers.

Recent versions of Emacs come with a start file for making emacs act like a syntax-
directed editor that understands (some of) Perl's syntax. See perlfaq3.

Users of vi should also look into vim and gvim, the mousey and windy version, for coloring
of Perl keywords.

Note that only perl can truly parse Perl, so all such CASE tools fall somewhat short of
the mark, especially if you don't program your Perl as a C programmer might.

The Perl Profiler
If you wish to supply an alternative debugger for Perl to run, invoke your script with a
colon and a package argument given to the -d flag. Perl's alternative debuggers include a
Perl profiler, Devel::NYTProf, which is available separately as a CPAN distribution. To
profile your Perl program in the file mycode.pl, just type:

$ perl -d:NYTProf mycode.pl

When the script terminates the profiler will create a database of the profile information
that you can turn into reports using the profiler's tools. See <perlperf> for details.

Debugging Regular Expressions

"use re 'debug'" enables you to see the gory details of how the Perl regular expression
engine works. In order to understand this typically voluminous output, one must not only
have some idea about how regular expression matching works in general, but also know how
Perl's regular expressions are internally compiled into an automaton. These matters are
explored in some detail in "Debugging Regular Expressions" in perldebguts.

Debugging Memory Usage

Perl contains internal support for reporting its own memory usage, but this is a fairly
advanced concept that requires some understanding of how memory allocation works. See
"Debugging Perl Memory Usage" in perldebguts for the details.

Use perldebug online using onworks.net services