When you think "unique" and "frequency", think of a hash. Its keys are always unique, and you can use its values as a place to store the number of times each key shows up:

for my $word (@list) {
  $frequency{$word}++;
}
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Now the %frequency hash holds each word (but only once!) and $frequency{$some_word} is the number of times $some_word appeared in @list.