I don't like the use of can there. Why not a simple callback with override:

sub param {
  my ($self, $request, $params) = @_;

  $params = $self->SUPER::params($request, $params);

  return { $self->custom_params($request, $params), %$params };
}

sub custom_params {
    # override this in subclasses to return a list of additional
    # key/value pairs to be added to your global namespace
    return ();
}
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It simplifies the code tremendously, and it's also much faster.

Introspection is expensive. Avoid it when possible.

-- Randal L. Schwartz, Perl hacker
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