Elements of @_ are aliases to the arguments, i.e. when you change $_[0] directly, you change the original argument, which in this case happens to be $num, respectively %hash.

See perlsub:

The array @_ is a local array, but its elements are aliases for the actual scalar parameters. In particular, if an element $_[0] is updated, the corresponding argument is updated (or an error occurs if it is not updatable). 

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,

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In reply to Re: The @_ array and the lexical variables by choroba
in thread The @_ array and the lexical variables by G4143

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