Why? Because that's the right syntax. The canonical invocation of a coderef is:

&{  THING_THAT_CONTAINS_A_CODE_REF }( @ARGS ) # (@ARGS) optional
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Which would be your final line. You may drop the curly braces around THING_THAT_CONTAINS_A_CODE_REF if and only if the THING is a simple scalar variable. Yours is not.

As an alternative syntax, you may use

THING_THAT_CONTAINS_A_CODE_REF -> ( @ARGS )
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which would be

$hash{test}->()
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in your case. That last syntax is due to yours truly challenging chip to get it in during the final perl 5.004 gamma release, which he did. You're welcome. {grin}

-- Randal L. Schwartz, Perl hacker