You are right about having to protect the values in @tmpl. Once you change the word DOMAIN to the first @subs value, in your original code you have changed your template itself and can no longer substitute for DOMAIN -- in your case you now need to substitute for "itchy.foo"

Try this sample code, where I substituted for the $j variable name something a bit more descriptive, and before each potential replacement saved each template string to a new "str_for_subbing" (and made up an "input" for the template file so I could see some real output).

#!/usr/bin/perl

use strict;

my @subs = qw(itchy.foo scratchy.bar homer.bab);

my @tmpl = ("To: webmaster\@DOMAIN",  "Dear Sir or Madam:", "You have 
+won \$1,000,000!");

my ($sub, $str, $str_for_subbing);

for $sub(@subs) {
  open(FH,">virt.$sub") or die;
  
  # Assume @templ in your code should have been @tmpl 
  # -- good reason to use strict!
  
  foreach $str (@tmpl) {
    $str_for_subbing = $str;
    $str_for_subbing =~ s/DOMAIN/$sub/g;
    print "$str_for_subbing\n";
    print FH "$str_for_subbing\n";
  }
  close(FH)
}
[download]

If you don't assign each $str to a $str_for_subbing, you do change the value of $str in the template on the first substitution for DOMAIN, which was not your intent. Substituting in a "copy" of each string saves you this grief.

Live in the moment.