# Warning, untested code below.
for (@templ) {
    /^(\w+)\@DOMAIN$/ and print FH "$1\@$i"
        or print FH;  # implicitly prints $_ as usual
}

# Or if you prefer "if":

for (@templ) {
    if (/^(\w+)\@DOMAIN$/) {
       print FH "$1\@$i"; 
    }
    else {
       print FH;
    }
}