I was curious and you made me check. There's a number of releases with incompatible changes listed if you google for it. For example a whole list at https://perldoc.perl.org/5.12.0/perldelta#Other-potentially-... So no, there's been quite a few.
Yes, most of them relatively minor though, which is why the OP has never stumbled across one.
(I found one once-- it turned out one coder had invented his own hash slice syntax. It wasn't supposed to work, but it did, until a particular upgrade...).
In general, Perl has been traditionally committed to backwards compatibility, but not fanatically so-- there is a deprecation cycle that can be used to remove the more problematic things.