# 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;  
}

##</code><code>##

# 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;
    }
}