In real life, those lines were paths, and I wanted all of then ending in a single slash.

Ah, I see. You want canonpath from File::Spec ;-) (doesn't add the trailing slash, but there are multiple possible solutions for that as well)

And this is admittedly not the prettiest solution, but it works: s{(?| /* (\n) | /+ (\z) | [^/] \K (\z) )}{/$1}mgrx. Or just split the input string on \n...