That's it almost. By saying "filter program" I think the OP means the words have to be piped to the filter program. Filters usually take STDIN as input.

Also, from the description of the OP I assume his program will deal with different filters, set by (e.g.) commandline parameters.

So

my $newword = `sed $word`;
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should become

my $newword = `echo '$word' | $filterprogram`;
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Where $filterprogram has to be set somewhere else in the script.