I believe one of its intended purposes was to allow for predefining the length of an array. So the '#' is symbolic of, 'number' of elements you want it to have:

use Data::Dumper;
my @test;
$#test = 20;
print Dumper(\@test);
print $#test , "\n";
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When used in a void syntax (verbiage?) it simply returns the current number of elements.

That doesn't mean I like it anymore or less then you do.