I've started playing around with perl just a week ago

Some comments unrelated to your question:

• Avoid using global variables. Use lexical variables for file handles.

  open my $fh, "text_file_with_words.txt" or die;
  while (my $line = <$fh>) {   
     ...
  }
  [download]

• $! contains an error message when open fails.

• I prefer to use the three argument form of open. It has fewer possible surprises.

  open(my $fh, '<', $qfn)
      or die("Can't open input file \"$qfn\": $!\n");
  [download]

• (my $var1, my $var2)
  can be written as
  my ($var1, $var2)