Further to BrowserUk's Re: Problems with a regex?:
The  /^\Q$stack$/ regex is equivalent to a  $string eq $_ string equality test. Also,   List::MoreUtils::any   is, IMO, better than the OPed usage of grep from the standpoint of self-documentation and maintainability (and it's also terse enough that it may not need to be encapsulated in another function).

>perl -wMstrict -le
"use List::MoreUtils qw(any);
 ;;
 my @ra = qw(foo BAR baz quux boff);
 ;;
 for my $string (qw(BAR bar xyzzy)) {
   print qq{'$string' is },
         check_stack($string, \@ra) ? '' : 'NOT ',
         qq{in (@ra)};
   }
 ;;
 sub check_stack {
   my ($string, $array_ref) = @_;
   return any { $string eq $_ } @$array_ref;
   }
"
'BAR' is in (foo BAR baz quux boff)
'bar' is NOT in (foo BAR baz quux boff)
'xyzzy' is NOT in (foo BAR baz quux boff)
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