I think the problem is that Regexp isn't yet a consolidated type. It is technically a bless()ed reference to the mystical Regexp package, but there's also some magic going on behind the scenes (as demonstrated in Juerd's node above). So it can and will behave like a base type, but also has the power of being bless()ed (are these the first steps towards a more Ruby-like language? ;-). See here for a nice summary on the discussion of what a Regexp really is.
HTH

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broquaint