Step one is the naive approach of searching with a regular expression:

@matches = grep /$str_to_find/, @Output;  # WRONG

This gives you those strings that did match. But it breaks once $str_to_find contains data that is parsed as regexp metacharacter, since these modify the behavior of the match -- you can get both false positives and miss on true matches.

So the solution is to quotemeta $str_to_match first, and match against that:

$quoted_str_to_match = quotemeta $str_to_match;
@matches = grep /$quoted_str_to_match/, @Output;
[download]

You can read up on quotemeta and \Q in perlre.

Update: D'oh! TMTOWTDI, but friedo's way is much simpler in this case :-)

Still, quotemeta/\Q are good to know about.