Just because an external entity accepts both forms doesn't mean that two perl-internal functions should return the value of the same external entity in two different forms.

You still wouldn't complain if caller() returned C:/foo/bar or C:\FoO\BaR, depending on if it's April 1st or not? ;)

Seriously, the problem is that in the current implementation, it's unnecessarily complex to check the return value of caller() in a platform-independent way. File::Spec->catfile() offers this canonical way, but doesn't work in this case.