You are creating two hash references which refer to empty hash arrays. There is no such thing as an undefined hash reference. Maybe you could do one or more of:

{ this_rule=>'some value' } # These rules
{} # no rules
0 or undef # no rules (! UNIVERSAL::isa($var, 'HASH') and false)
{ all_rules=>1 } #all rules
1 # all rules ( ! UNIVERSAL::isa($var, 'HASH') and true)
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