I tried looking at the module source myself ...

In case you don't mind modifying the sources... this is what a quick grep turned up (in RPC2.pm / sub serve {...}):

    my $eval = eval { $result = &{ $methods->{$method} }(@{ $call->{'v
+alue'} }) };
    if ($@) {
       ...
    }
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Presuming that the $self available within that serve method is what you need (not sure... but why not try?), you could add it to the argument list yourself, i.e.

    my $eval = eval { $result = &{ $methods->{$method} }($self, @{ $ca
+ll->{'value'} }) };
                                                         ^^^^^
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Just an idea... (untested)