Session Alias

Basic question here but I can't seem to find the answer (I am looking
for)...

How do I determine the alias of the session that sent and event?  For
example, in this handler:

sub scanning {
    $_[HEAP]->{speed_sel}->setSelectedIndex( 0 );
}

I want the SENDER's alias.  However, sessions do not have an obvious way
of coughing up their alias (no public method).  What am I missing?

Thanks,

John