Besides readability and safety upon forbidden keywords ? Not much, i guess.
TIMTOWTDI applies. Of course, you could always just - when in need of speed:

# my constant FOO
sub FOO () { 'bar'; }
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which would provide you with a compile-time constant and not use the "glob syntax" you mentioned. Last I saw on p5p, constants had this bad habbit of eating large chunks of ram. So abusing them in such circumstances would not be advisable. YMMV($code).