Understanding this is part of understanding how references work.

You are not changing the actual values in either the function or the original data structure. Those values are a pointer saying, The real data is over there. Instead you're manipulating the value of what the reference is pointing to. Since both are pointing there, the side-effect is that both see the change.

To achieve the effect that you are looking for, you need to change the value of the hash by creating new references. Like this:

sub push_1 {
    my %hash = @_;
    for (keys %hash) {
        $hash{$_} = [ @{ $hash{$_} }, 1 ];
    }
}
[download]

UPDATE: It became clear in chatter that I should make it precise what references are involved. The references that I'm talking about are the anonymous array references that are the values of the hash.

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