The output (below) shows that the attempt to assign a new hashref fails silently.

No, it works:

sub assign_new_href {
    my $href = shift;
    $href = { new_key => 4 } ;
}
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Within that function, you do assign a new hash reference to the lexical $href. However it doesn't modify the hash to which $href refers because that's not how references work. Think of $href as a container. You've dumped out its contents to replace those contents with something else. What you want to do is replace the contents of its contents with something else:

sub assign_new_href {
    my $href = shift;
    %$href = ( new_key => 4 );
}
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