$self->{YYYY}->function_name();
is
( $self->{YYYY} )->function_name();

So first find out what $self->{YYYY} contains. If it's an object, printing it out (or printing out ref($self->{YYYY})) should tell you what class it belongs to. Alternatively, if it's a file handle, it's automatically associated with the Filehandle or IO::File package