all the out-of-date OS support and huge swaths of other historical gunk
Unfortunately in relative terms, that accounts for mostly nothing.
99% of the complexity of the perl interpreter/compiler/runtime is due to its initial design. A clear example of premature and abusive optimization... maybe it made sense twenty years ago but nowadays it is just a heavy burden stopping perl 5 development in anything but trivial matters, and specially in getting new blood into it.
Anybody wanting to advance perl 5 seriously, should consider starting from scratch!