Try this. Save it as yourperl/site/lib/Devel/Trace/Remote.pm

Updated: Improved the cleanup to allow reconnects.

ppackage Devel::Trace::Remote;
use strict;
use warnings;
use IO::Socket;
use threads;
use threads::shared;
use Thread::Queue;

my $connected :shared = 0;

my $Q = new Thread::Queue;

sub DB::DB {
    return unless $connected;
    $Q->enqueue( join ' : ', caller );
    return;
}

async {
    my $server = IO::Socket::INET->new(
        LocalHost => 'localhost:54321',
        Listen => 1,
        Reuse => 1,
    ) or die $!;
    $server->autoflush;
    while( my $client = $server->accept ) {
        $connected = 1;
        while( $_ = $Q->dequeue ) {
            print $client "$_\n\r" or last;
        };
        $connected = 0;
        $Q->dequeue while $Q->pending;
    }
}->detach;

1;
[download]

Then start your script using perl -d:Trace::Remote yourscript. When things start going awry, telnet into localhost:54321 and you will get output along the lines of:

##pkg  file      lineno
main : ack1.pl : 21
main : ack1.pl : 22
main : ack1.pl : 21
main : ack1.pl : 22
main : ack1.pl : 21
main : ack1.pl : 22
main : ack1.pl : 21
main : ack1.pl : 22
main : ack1.pl : 21
main : ack1.pl : 22
main : ack1.pl : 21
main : ack1.pl : 22
main : ack1.pl : 21
main : ack1.pl : 22
main : ack1.pl : 21
main : ack1.pl : 22
main : ack1.pl : 21
[download]

With nothing connected, it will have minimal impact on the process. (For cpu-bound processes about 4 1/2 times slower. Much less for a IO-bound process. When you are connected, expect the slowdown to roughly double.) You may also see some memory growth if the program is is a tight loop (generating output faster than telnet can receive it.

Many enhancements are possible, but this should allow you to get a feel for what is going on inside the code without to much impact on the normal running. It may end up on CPAN if you find it useful.