my $infilename = "/etc/postfix/virtual";
open my $infile, "<", $infilename or die "Can't open $infilename to read: $!\n";
open my $outfile, ">", "$infilename.new" or die "Can't open $infilename.new to write: $!\n";
local $_;
while(<$infile>) {
    print $outfile $_;
    print $outfile "$user\@$dom\t$user\n" if /^\Q$dom\E\b/;
}
close $infile;
close $outfile;

rename("$infilename.new", $infilename) or die "Couldn't replace $infilename with $infilename.new: $!\n";