my original code (before i changed the layout of the file) looked like this:

my $datafile = '/etc/postfix/virtual';
my $req_addr = "$username\@$domain";
my ( $name, $atdomain ) = $req_addr =~ /^([\w.-]+)(@[\w.-]+)$/ ;
open FH, $datafile or die;
my @virtual = <FH> ;
close FH ;
open FH, ">$datafile" or die;
my $wrote_it = 0 ;
for ( @virtual )
{
  print FH $_ ;
  if ( $_ =~ /$atdomain/ && !$wrote_it )
  {
          print FH "$req_addr\t$username\n" ;
    $wrote_it++ ;
  }
}
close FH ;
[download]

Which would edit the /etc/postfix/virtual file if it looked like this:

aaaa.com        VIRTUALDOMAIN
@aaaa.com        postmaster


bbbb.com     VIRTUALDOMAIN
@bbbb.com    postmaster

cccc.com    VIRTUALDOMAIN
@cccc.com   postmaster
[download]