#  $fh1  $fh2
                                     #  ----  ----
open(my $fh1, '>', $file1) or die;   #     1
open(my $fh2, '>', $file2) or die;   #           1
for (@data) {                        #
   print $fh1 "$_\n";                #     1
   print $fh1 "$_\n";   # Oops       #     1
}                                    #
close($fh1);                         #     1
close($fh2);                         #           1
                                     #  ----  ----
                                     #     4     2   -> no warnings