"I'm confused, I thought this would work ..."

Using -MO=Deparse,-p will show you how Perl parses the code and why it doesn't work:

$ perl -MO=Deparse,-p -e 'sub ff($$){ qq{@_} } my %f = ( 12 => ff 1,2,
+ 34 => ff 3,4 );'
Too many arguments for main::ff at -e line 1, near "4 )"
-e had compilation errors.
sub ff ($$) {
    "@_";
}
(my(%f) = (12, &ff(1, 2, 34, ff(3, 4))));
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On the last line of the output, you'll see &ff(1, 2, 34, ff(3, 4)) has 4 arguments - your prototype specifies 2.