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Can someone explain why this makes a new hash and returns a reference to it:

sub hashem { +{ @_ } }
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A hash is just an associative array, ie the following will work as a hash but may look like an array to you:

my %hash = ( a, 1, b, 1, c ,1 );

$hash{a} prints 1 as does $hash{b} and $hash{c};

The { } references @_ which is essentially the same as

%hash = \%hash;

The + sign forces an anonymous hash contructor.