There's a much simpler way to achieve this; though it does have a bit more overhead, it should only be significant for "large" hashes, since it involves copying all the elements.

sub assign_ME_problem
{
    my( $ontCategory, $ontNumbering, $mapwd ) = @_;
    local( %problemClassProps ) = %problemClassProps; # localize a cop
+y
    markFail( $mapwd, $ontCategory, $ontNumbering );
    %MEClassProps = %problemClassProps; # copy to your hash
    1
}
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