The approach that occurs to me would be to put your keyword tests into a subroutine and use the each function to test the sentences in a way that makes the index / sentence number easily accessible.

while (($key, $value) = each @sentence) {
   if (has_one_or_more_keywords($value)) 
      print "$key: $value\n";
}
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I second the suggestion to consider an HTML parsing module. You may also want to consider replacing your sentence and word splits with a more sophisticated grammar for parsing sentences and words using a module like Parse::RecDescent. For example, the period character is not a sentence terminator when used in an ellipsis, as a decimal point in a number, etc.