my @reg_addrs = qw(bob@domain1.com bob@domain2.com);
open FH,"<",$datafile or die "Could not read virtual file";
my @virtual = <FH>;
close FH;
for my $reg_addr (@reg_addrs) {
    my ($name,$domain) = split /@/,$reg_addr;
    my ($i) = 0;
    for my $entry (@virtual) {
        ++$i;
        if ($entry =~ /\Q$domain\E/) {
            splice @virtual,$i,0,"$reg_addr\t$user\n";
            last;
        }
    }
}
open FH,">",$datafile or die "Could not write virtual file";
print FH @virtual;
close FH;