I'm going to go out on a limb here and guess it isn't Perl

A questionable assumption. Define Perl.

One likely scenario is software compiled with the PCRE library. A growing number of applications do so these days. I know Apache will use it if it's there (although I don't know what for... maybe mod_rewrite these days?) and so will Postfix (and I'm a heavy user of it and very happy it's available).

A pure-Perl scenario I can envisage is a callback/library situation, where you get to construct an object or otherwise pass a qr/.../ where the design (admittedly somewhat brain-damaged) only admits positive matches. The not, ! and unless solutions in the other responses does one no good if you can't manoeuvre around the code that's doing the matching.

So please don't be so hasty in dismissing a person and telling them to go away. I agree the OP should have provided more information, but since it was posted here, maybe it has something to do with Perl?