my $datafile = '/etc/postfix/virtual';
=pod
# This bits a mess?
# First you composite the username and domain together
my $req_addr = "$username\@$domain";

# Then you use a very iffy regex to break them apart?
my ( $name, $domain ) = $req_addr =~ /^([\w.-]+)(@[\w.-]+)$/ ;

=cut

# so assuming that $name and $domain contain the right information
my ($line, $mark, $rest);
open FH, "+<$datafile" or die "Open $main::FH failed; $!\n"; #1
$line = <FH> while $line !~ /^$domain$/o;                    #2
$mark = tell(FH), $line = <FH> while $line =~ /$domain/o;    #3
@rest = ($line, <FH>);                                       #4
truncate( FH, $pos );                                        #5
print FH "$name\@$domain\t$name\n";                          #6
print FH "@rest";                                            #7
close FH or die warn "Error closing $main::FH; $!\n";        #8