You are not passing it by reference: you are just passing the hash.

What happens is that @_ in the sub flatten out the hash so each keys and values go into the @_ array and you assign it to a scalar $existing_users

You pass by reference.. passing a reference: process_users(\%results) or process_users($results_reference) and inside the sub you just grab the first argument arrived in (the flattener) @_ and you know it is a reference, so:

sub process_users(
    my $result_ref = shift;
    # dereference it if needed
    foreach my $key ( %$result_ref ){
    ...
)
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L*