Maybe a more perl-guts competent poster can comment on the technical benefits of one versus the other, but I see this as the Perl motto: TIMTOWTDI.

Personally, I'd use them exactly as you describe. The first (to me) is more readable as to what I'm doing and I'd use it to assign a single value. I'd use the second form in the exact use case you point out - 'push' values to the existing structure.