Because that is a snippet, we can't tell where you're defining your variables. Where is your definition of "stopwords"? If it is a list, $stopwords is a different variable from @stopwords (one is a scalar, or single value, the other is a list).

Based upon my best guess, you need a grep, not a regexp.

Grep is used to search for a value within a list, so you'd want something like:

unless ( grep { / $word / } @stopwords ) {
  push @lessWords, $word;
}
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But please click the link on grep to explore WHY that is the case.

Update: Fixed grep logic based upon OP.