Yes, though Tie::File isn't really an "additional module," since it's now part of the 5.8.0 distribution. Here's how you could do it w/o Tie::File:

#! /usr/bin/perl

use strict ;
use warnings ;
$|++ ;

my $datafile = 'test_a.dat' ;

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

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

open FH, $datafile or die "Couldn't read $datafile: $!" ;
my @virtual = <FH> ;
close FH ;

open FH, ">$datafile" or die "Couldn't write $datafile: $!" ;

my $wrote_it = 0 ;
for ( @virtual )
{
    print FH $_ ;
    if ( $_ =~ /$domain/ && !$wrote_it )
    {
        print FH "$req_addr\t$username\n" ;
        $wrote_it++ ;
    }
}

close FH ;

__END__
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Those who know that they are profound strive for clarity. Those who
would like to seem profound to the crowd strive for obscurity.
            --Friedrich Nietzsche

Thanks very much for your time mate. That worked a treat. Big Up DamnDirtyApe! :-)