But I recall looking at the code and thinking that the creation of the 'true' and 'false' values should've been done in Perl.

JSON::XS does create true and false in Perl.

our $true  = do { bless \(my $dummy = 1), "JSON::XS::Boolean" };
our $false = do { bless \(my $dummy = 0), "JSON::XS::Boolean" };

sub true()  { $true  }
sub false() { $false }
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So many the problem can be solved by upgrading the module the OP has installed? In any case, the docs say "This module is not guaranteed to be thread safe".