Why do I need to localize $@ before using eval?

The reason to say local $@ before calling eval is to avoid stepping on your caller's $@. It's rude for a subroutine to alter any global variables (unless that's one of the stated purposes of the subroutine). This isn't really an issue with top-level code (not inside any subroutine).

Also, on older Perl's, any eval called during object destruction would clobber the global $@ (if the object was being destroyed because an exception was being thrown from an eval block) unless $@ was localized first. This was fixed in 5.14.0, but many people are still running older Perls.

The Try::Tiny module documentation gives the rationale (as well as providing an alternative):

When you run an eval block and it succeeds, $@ will be cleared, potentially clobbering an error that is currently being caught. This causes action at a distance, clearing previous errors your caller may have not yet handled. $@ must be properly localized before invoking eval in order to avoid this issue. More specifically, $@ is clobbered at the beginning of the eval, which also makes it impossible to capture the previous error before you die (for instance when making exception objects with error stacks).