Or, hide that in a subroutine is_regex() or something similar.

Like this?

BEGIN {
   if (!defined(&re::is_regexp)) {
      package re;
      my $re_class = ref qr//;
      *is_regexp = sub($) {
         local *__ANON__ = 'is_regexp';
         return UNIVERSAL::isa($_[0], $re_class);
      };
   }
}
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Using your philosophy to avoid magical constants, it checks if re::is_regexp is defined rather than checking $] for 5.10.

BEGIN and the prototype duplicates the behaviour of 5.10's re::is_regexp.

It uses isa instead of ref so regexes can be made detectable even if there's a need to rebless the regex (by adding Regex to that class's @ISA). It's not an invitation for non-regexes to pretend to be regexes.