The documentation of the space_after option says that it "will add ... extra whitespace after the , separating key-value pairs and array members." However, it doesn't actually do this.
use warnings;
use strict;
use Cpanel::JSON::XS ();
use JSON::PP ();
print Cpanel::JSON::XS->new->utf8->space_after->encode({x=>[1,2]}), "\n";
print JSON::PP->new->utf8->space_after->encode({x=>[1,2]}), "\n";
__END__
{"x": [1, 2]}
{"x": [1,2]}