Your code requires Gearman::XS::Client to bless the object into the class 'My::Gearman::Client'. Gearman::XS::Client does not necessarily do this. Although the common way to write a constructor is something along the lines of:

sub new {
  my $class = shift;
  bless {}, $class;
}
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Not all modules will do it this way, and will bless into some constant class name. That appears to be what is happening. You can try filing a bug report (hopefully with a patch) that the module cannot be sub-classed, or you can make your library a delegator instead of a parent class.