Good point. But this means that either @INC was explicitly changed in some BEGIN section, or the Perl installation itself was broken, because even without setting any PERL5LIB variable, @INC should at least contain the path to the core libraries.

To check the installation, one could do a

perl -lwe "print @INC"
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on the Windows command line. On my system, this yields

c:/perl/site/libc:/perl/lib.
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(and, no, I have no idea where the dot after lib comes from, but on Windows, LIB and LIB. are presumably the same anyway). If @INC is empty here too, the installation is broken (but then Perl should find *no* Core modules at all). So, more likely, there must be some BEGIN section which resets @INC....