Maybe it has something to do with print being on the same line?
Weird.

% perl -e '%h = (q => 1, w => 2, x => 0, y => 2);\
 print join ",", reverse %h = reverse %h;'
w,2,x,0,,2,q,21

% perl -e '%h = (q => 1, w => 2, x => 0, y => 2);\
 %h = reverse %h; print join ",", reverse %h;'
w,2,x,0,q,21
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