Editing in the middle of a file either requires creating a copy of the file with the new output and renaming back that over the old copy, or a relatively complex approach to copy the back of the file down X bytes after inserting your string of X bytes. For the first approach, something like this would do:

my $infilename = "/etc/postfix/virtual";
open my $infile, "<", $infilename or die "Can't open $infilename to re
+ad: $!\n";
open my $outfile, ">", "$infilename.new" or die "Can't open $infilenam
+e.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 $infil
+ename with $infilename.new: $!\n";
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The simple way to use the other approach would be to use the Tie::File module.

Makeshifts last the longest.