You are right that it is likely that the bug is in MooseX::Declare or one of its dependencies. I would like to find a way to reproduce this without resorting to other complicated modules like Regexp::Grammars (even though this module is pure Perl).

Update: Other segmentation faults have already been described that are triggered by RG’s extensive use of the regex engine, so it’s not that unlikely that the culprit is in the regex engine.