#! /usr/bin/perl

use strict ;
use warnings ;
$|++ ;

use Tie::File ;

my $datafile = 'test_a.dat' ;

my $username = 'terry' ;
my $req_addr = 'tag@foo.com' ;

my ( $name, $domain ) = $req_addr =~ /^([\w.-]+)(@[\w.-]+)$/ ;

tie my @virtual, 'Tie::File', $datafile or die $! ;

my $row = 0 ;
$row++ until $virtual[$row] =~ $domain ;
$virtual[$row] .= "\n$req_addr\t$username\n" ;

untie @virtual ;

__END__