Your code does several things more than you describe. In addition to removing (case-insensitive) duplicates, it also has some other criteria for elimination, which should not be inside the foreach.

To duplicate your code functionally, we have to:

1. Remove non-words (you missed a closing / in that pattern, by the way)
2. Remove words shorter than 4 chars
3. Remove words that appear more than once (your technique suggests that the list is sorted)
4. Remove words that appear in @exclude

Ok, here we go!

# given @words and @exclude
my %seen;
@seen{@exclude} = (1) x @exclude;
@words = grep {
    (! /\W/) and (length() >= 4) and ($seen{$_}++ == 0)
} @words;
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We pre-load %seen with a flag for each word in @exclude. Then, as we're looking through @words itself, we mark each element as seen as well.