I guess for the same reason OpenSSL needs Perl, to generate some files.

IIRC most notable the ASM files are generated via Perl.

Update: Here is a link to some Perl BoringSSL stuff: https://boringssl.googlesource.com/boringssl/+/master/crypto...

Worse: why does it need Go?

Perl is used to generate the final assembly files so they can be adapted between different x86_64 variants/systems/assemblers.