# Warning, untested code below.
  
for my $j(@templ) {
    my $k = $j;
    $k =~ s/^(\w+)\@DOMAIN$/$1\@$i/;
    print FH $k;  
}

# Or:

foreach (@templ) {
    my $j = $_;
    $j =~ s/^(\w+)\@DOMAIN$/$1\@$i/;
    print FH $j;  
}