I'm not arguing against your statement: "The fat comma doesn't follow different rules of autoquoting than hash keys do.".

However, the target of autoquoting may be handled differently prior to autoquoting.

Here's an example with constants:

$ perl -wE 'use constant XXX => q{a}; my %x = (XXX() => 123); say $x{X
+XX()};'
123

$ perl -wE 'use constant XXX => q{a}; my %x = (XXX() => 123); say $x{+
+XXX};'
123

$ perl -wE 'use constant XXX => q{a}; my %x = (+XXX => 123); say $x{XX
+X()};'
Use of uninitialized value in say at -e line 1.

$ perl -wE 'use constant XXX => q{a}; my %x = (+XXX => 123); say $x{+X
+XX};'
Use of uninitialized value in say at -e line 1.
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Note how both XXX() and +XXX can be used as the hash key but only XXX() works on the LHS of the fat comma.

This example is run under 5.12.0 but I recall the same behaviour under 5.8 (definitely) and 5.6 (probably).

I have a vague recollection that there's another instance of this type of behaviour but I can't think what it is right now.