Because you are making a copy of the reference, and then you modify the copy. Either change the alias, or modify what the reference is pointing to:

sub assign_new_ref {
    $_[0] = {new_key => 4};  # Change alias
}
sub assign_new_ref {
    my $href = shift;
    %$href = (new_key => 4); # Change what the reference is pointing t
+o
}
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Code is untested.