On this occasion, it appears that the problem is internal to Perl--with caveats.

The problem occurs in these two lines from win32\win32.c (line numbers are approximate (5.8.4) because I've added debug code):

1222:         pwbuffer = PerlDir_mapW(wbuffer);
...
1228:         path = PerlDir_mapA(path);
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Prior to these to lines the vars; path / wbuffer contain the bad paths being tested ( ' ', ' . . ' etc. ), but after they contain the fully expanded current working directory. They should not.

So, the problem is in the routines PerlDir_mapW() / PerlDir_mapA(). The stat being returned for non-existant directory paths is that of the current working directory.

But there, my trail grows cold. These functions are not functions, but #defines.

One path through the #defines in iperlsys.h has these resolved to:

#ifdef WIN32
#define PerlDir_mapA(dir)        dir
#define PerlDir_mapW(dir)        dir
#endif
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But that can't be right because then they would be unchanged and the problem wouldn't occur.

The other branch of the #if defined(PERL_IMPLICIT_SYS) in iperlsys.h resolves them to entries in a vtable.

#ifdef WIN32
#define PerlDir_mapA(dir)                    \
    (*PL_Dir->pMapPathA)(PL_Dir, (dir))
#define PerlDir_mapW(dir)                    \
    (*PL_Dir->pMapPathW)(PL_Dir, (dir))
#endif
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but search high'n'low, I cannot find out where these vtable entries are filled in?

It appears to be related to PL_curinterp and embedvar.h, but I tracked it back through about 6 levels of indirection before getting completely lost. It will take someone much cleverer than me to resolve this further.