But why on earth would Perl not return an error when opening (not opendiring) a directory?

As others have shown, there's nothing intrinsically invalid about open on a directory. However I think Perl is definitely capable of giving a more informative error message when you try to use a filehandle in a context that requires a directory handle. I'd recommend opening a github issue tagged as 'Wishlist' suggesting that there's an opportunity for improvement here.

Update: the issue now exists as #22394.

In reply to Re^3: "readdir() attempted on invalid dirhandle" error? by hv
in thread "readdir() attempted on invalid dirhandle" error? by Anonymous Monk

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