Instead of using length, which counts the line terminator as Marshall discovered, you could test with a regular expression for 5 or 6 characters between beginning and end of string anchors. The task could be done as a one-liner like this.

$ perl -ne 'print if m{^.{5,6}$};' all_words.txt >> trimmed_words.txt
$
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Change the >> to > if you are not appending to the file. I hope this is of interest.