If you can get your variable input with multiple words comma separated and quotes around, like:

'word1', 'word2', 'word3'

you can write the expression without quotes, like:

 Sum({<[word-set]-={$(word)}>}count)

Or, if your words are really words, i.e. - single word inputs (without any spaces in between), you can get this working even without single quotes, like:

word1, word2, word3