You are also wrong about \%. \% means the sub requires a hash, not a hashref.

>perl -le"sub f(\%) { print $_[0] }  f(\%h)"
Type of arg 1 to main::f must be hash (not reference constructor) at -
+e line 1, at end of line
Execution of -e aborted due to compilation errors.

>perl -le"sub f(\%) { print $_[0] }  f(%h)"
HASH(0x238be4)
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A hashref is what the sub receives.

There is no prototype that will enforce what you desire (a scalar, an array and an optional hashref). Prototypes aren't good at validation.