and delighting in the potential for unreadable code coming from using a conditional operator as an lvalue

It takes next to nothing to make the cond op unreadable, but using it as an lvalue itself doesn't make it unreadable.

($axis ? $x : $y) += $distance;
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I can't see why 1 ? my $c : my $d would be parsed that way, but 1 ? my $c : $d wouldn't.

It appears to be another case where heuristics enter into play.

my is a valid attribute name, so : my is presumed to be an attribute.

$d is not a valid attribute name, so : $d is taken to be the else expression of the conditional.

Untested, but parens should clear that up.

1 ? (my $c : shared) : ...    # shared is an attribute

   -vs-

1 ? (my $c) : shared          # shared is func call
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perl -MO=Deparse -e 'my $c; 1 ? $c : my $d; $d'

Note that conditionally executing my officially results in undefined behaviour.