In checking what I thought were reasonable assumptions, I found that JSON::PP's handling of booleans and JSON::XS's handling of booleans are simply incompatible.

use JSON::XS();
use JSON::PP();
JSON::XS->new()->encode( JSON::PP->new()->decode('[true]') );
__END__
encountered object '1', but neither allow_blessed nor convert_blessed 
+settings are enabled
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Note that even enabling convert_blessed() won't help as JSON::PP generates booleans that don't even have a TO_JSON() method. [And enabling allow_blessed() would just turn true/false into null.]

Both modules could use some work on dealing with booleans.