They are subtly different. The base "perl5" directory is for architecture-independent code, while the "perl5/x86_64-linux-thre­ad-multi" dir is for that specific interpreter ABI. In effect, pure Perl extensions go in the former, while XS extensions go in the latter.

The reasons for having this type of directory structure go back to "ancient times" where multiple systems could share a common network-mounted /usr or the same disk could be used with different machines having different processors. Perl adopted this convention and extended it to allow for other variations that produce incompatible ABIs, such as the availability or omission of threading support. For me this is a bit of history, and I would welcome corrections or further explanations from more senior monks.

In reply to Re: Misplaced local cpan modules by jcb
in thread Misplaced local cpan modules by Robert II

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