Check which cpan is being called. You are probably using the version from msys2, which is why it then looks for /usr/bin/tar.

On my system:

where cpan
C:\berrybrew\5.28.0_64_PDL\perl\bin\cpan.bat
[download]

If you are running the portable shell that comes with Strawberry Perl then it should put all the strawberry paths at the front of %PATH%. Perhaps double check that you are using that shell.

And FWIW, I have MSYS2 and several versions of Strawberry perl installed on my system (via Berrybrew) and they don't interfere with each other. That said, none of them are in my default path. I only ever start MSYS2 from its menu option, and strawberry perl via a bat file that prepends the paths (similar to what its portable shell does).