Here's my version:

use strict;

my $input  = "words.txt";
my $output = "output.txt";

open my $ifh, '<', $input  or die "can't open $input  : $!\n";
open my $ofh, '>', $output or die "can't open $output : $!\n";

while (my $line = <$ifh>) {
  chomp $line;
  my $word_size = length($line);
  if ($word_size == 5 || $word_size == 6) {
    print $ofh "$line\n";
  }
}

close $ifh;
close $ofh;
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