You didn't retrieve any information from the data structure, so you didn't test it at all. Try this:

perl -Mv5.14  -MJudy::L=Get,Set -e'
   my $i = 1<<63;
   my $j = -$i;

   my $judy;

   Set( $judy, $i, 4 );
   Set( $judy, $j, 5 );

   say( ( Get( $judy, $i ) )[1] );
   say( ( Get( $judy, $j ) )[1] );
'
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If you get different values, I was wrong. If you get the same value, each negative value is considered to the same as a very large positive value.

Without the hack, it outputs 5 twice. I suspect the hack only silences the warning.

Of course not.

No, it isn't obvious that the keys are 65 bits (or larger) in size. In fact, they pretty obviously aren't.