This helped me recently for example, when I was trying to use Net::SMTP with a server which required authentication, and it kept failing on me. Finally I used the debugger to step into the library, and saw the following code:

eval {
  require MIME::Base64;
  require Authen::SASL;
} or $self->set_status(500, ["Need MIME::Base64 and Authen::SASL todo auth"]), return 0;

So, I installed the Authen::SASL module and everything worked. While I would have preferred for the module to say it more explicitly (dying with a suggestive error or in the documentation), but still, the fact that I could look at the source and understand the problem is great.

To be specific: in one of my Perl scripts I wanted to send mail. The problem was that I didn’t have access to a SMTP server on that machine. After toying around with the example code of Net::SMTP::Server I concluded that it didn’t work and went off to find some lightweight solution. One of the more promising solutions was RunMail, however I was still uncomfortable about running binary code of unknown origin on the server. As a results I fired up telnet and the Perl debugger and stepped into the library source code. There I found the following part:

    # Loop through the recipient list.
    foreach $target (@{$self->{TO}}) {
 my $rr;
 my $domain = /@(.*)/;
 my $res = new Net::DNS::Resolver;
 my @mx = mx($res, defined($1) ? $1 : hostdomain);

I promptly replaced the line so that $1 would be set, and voila, it works!

    # Loop through the recipient list.
    foreach $target (@{$self->{TO}}) {
 my $rr;
 $target =~ /@(.*)/;
 my $res = new Net::DNS::Resolver;
 my @mx = mx($res, defined($1) ? $1 : hostdomain);

Now, if this would have been a binary library, I would had to: (a) get the source code (b) set up a build environment (c) compile from source (to get the debugging symbols) and (c) figure out how to integrate the debugging version with the release version such that I get a breakpoint where I need to and I get the debugging symbols. And I didn’t even speak about the rich introspection capabilities of scripting language debuggers!

Update: the actual problem was in Net::SMTP::Server::Relay, not in Net::SMTP::Server.

Update: below you can see the full code of the module (since it is rather short). The part shown with bold is the one which needed to be changed:

package Net::SMTP::Server::Relay;

require 5.001;

use strict;
use vars qw($VERSION @ISA @EXPORT);

require Exporter;
require AutoLoader;
use Carp;
use Net::DNS;
use Net::Domain qw(hostdomain);
use Net::SMTP;

@ISA = qw(Exporter AutoLoader);
@EXPORT = qw();

$VERSION = '1.1';

sub _relay {
    my $self = shift;
    my $target;
    
    # Loop through the recipient list.
    foreach $target (@{$self->{TO}}) {
 my $rr;
 $target =~ /@(.*)/;
 my $res = new Net::DNS::Resolver;
 my @mx = mx($res, defined($1) ? $1 : hostdomain);
 
 next unless defined(@mx);
 
 # Loop through the MXs.
 foreach $rr (@mx) {
     my $client = new Net::SMTP($rr->exchange) || next;
     
     $client->mail($self->{FROM});
     $client->to($target);
     $client->data($self->{MSG});
     $client->quit;
     
     last;
 }
    }
}