What's really weird is that say scalar (my @a = ...) behaves differently from my @a = ...; say scalar @a;

I personally believe that actually:

$ perl -wMstrict -E 'say "@{[q|aaa| =~ /./g]}"'
a a a
$ perl -wMstrict -E 'say "@{[q|aaa| ~~ /./g]}"'
1
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FWIW, I realized this while writing the code for Finding differences in binary files: the line that starts with my @l was originally unnecessarily broken in two statements, the first of which was a match with ~~: then I thought I could condense them into one, but for some reason "it didn't work" - until I changed the operator to =~ "just to be sure," that is...