By "reverse grep", I take it that you mean you want to loop over a list of regexes with one string, rather than loop over a list of strings with one regex. As far as Perl is concerned, these are the same.

Your difficulty comes from thinking of Perl's grep as being the same as the command line grep. This is not the case. In Perl's grep, the conditional is not necessarily a pattern match; it is any arbitrary Perl expression. For example, you could do something like this: @results = grep { $_ & 1 } (1 .. 10) to get a list of odd numbers between 1 and 10.

Thus, you can do any filtering you want with grep, with the proper conditional expression. What you're looking for could be written something like this (with some regexes I made up):

my $url = 'http://www.perlmonks.org/';
my @regexes = ('/private(?:/|$)', 'python');
if (grep { $url =~ $_ } @regexes) {
  # handle bad URL
} else {
  # handle good URL
}
[download]

As suggested in another response, you'd probably want to precompile the regexes with qr// for efficiency.