first, please use code tags. As for your question of breaking out, check the return value of system ($?). Most unix commands return 0 upon success and something else upon failure (not quite sure what the failure codes are for adduser but I get a 9 when you try to add an all ready existing user).

#!/usr/bin/perl -w

use strict;
use Getopt::Long;

my ($username, $password, $domain);
my $result = GetOptions("username=s" => \$username, 
                        "password=s" => \$password,
                        "domain=s" => \$domain );

system("/usr/sbin/adduser $username -g 100 -s /bin/false -d /home/$use
+rname -p $password");

if( $? >> 8 == 0 ) {
   # success 
   open (VFILE, ">> virtual"); # you should check for failure here
   print VFILE "$username\@ap3k.com\t$username\n";
   close (VFILE);
   # Do you need to check this?
   system ("postmap /etc/postfix/virtual");
} else {
   print STDERR "bailing with ret code of ", $? >> 8, "\n";
}
[download]

see system on interpretting return codes (that shifty bit stuff).

-derby