As jeffa points out use CGI::Carp and look in the server's log files for the errors. Additionally you can use CGI::Carp qw/ fatalsToBrowser / if you don't have access to the server's log files.

Other techniques you can use involve some sort of trapping of errors and sending them out as html. An example:

#!/usr/bin/perl -w
#################################3
use strict;
use CGI;
use DBI;

my $q=new CGI;

my $dbh=DBI->connect("DBI:mysql:dbhost=localhost",
                     "dbuser","secret")
    or scream_and_die($q,$DBI::errstr);

#
# more code...
#

exit(0);

sub scream_and_die {
   my ($q,$str)=@_;

    print $q->header,$q->start_html(-title=>"UH-OH!"),
              $q->p("An error occured!"),
              $q->p($str),$q->end_html;

    exit(0);
}
[download]

The biggest problem with the example I give above is that if you use it in production you possibly can leak sensitive information if something bad happens.