elements/base/perl.bst: use buildflags from environment and bump to 5.28.1

Currently 'perl' doesn't use global buildflags which means it doesn't enable our optimizations or hardening.

Use optimize="none" as we already pass optimizations in our 'CFLAGS'.

This is the final nail in the coffin of #317 (closed) . After it's merged (alongside other already opened MRs) all things we build should follow our global, hardened buildflags (with per item exceptions when needed).

Also bump version to 5.28.1