I'm a little baffled about your "dictionary.txt" file. You say it is large, but you read it like this:

open(DICT, $dict) or die "Cannot open $dict because $!";
my $words = <DICT>;
my @words = split(/ /, $words);

close(DICT) or die "Cannot close $dict because $!";
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Normally, I'd assume that a "dictionary.txt" file would have a line-feed (or CRLF, depending on how the file was created) at the end of each word, rather than a space; this makes it easier to know how many words are present, and to add or remove words -- because counting lines is easy, and adding or (de)selecting lines is easier than adding or (de)selecting words within a single, very long line). But these issues might not be important to you, and you can do it whatever way suits you.

Just curious: is there a line-feed (or CRLF) at the end of that text file? If so, you don't seem to be doing anything about it (it remains attached to the end of the last word), and maybe this is affecting how that last word in the list is being handled when you put it into the submit_form request.