I was thinking for specific-version links, allowing doc://5.8.3/perlre, and making a drop-down in user settings to set your default.

There is a defined behavior for use on any string not matching /^perl/: assuming it is a function. Thus, Time::Local links to the documentation of the (nonexistant) function Time::Local; if you wanted the documentation of that module, you should have said Time::Local. (Core modules are on cpan.) I may change that to output some plain ol text, as it's pretty clearly an error. I may even extend the DWYMery to link to the POD for that module, though I'm worried that we'll get into places where we are unable to do what you meant: is doc://open documentation on the module open, or the function open? (Yes, there is one of each.)