Perl de-reference code reference variable

Guys,

May i know how can we de reference the code reference variable.?

my $a = sub{$a=shift;$b=shift;print "SUM:",($a+$b),"\n";};
print $a->(4,5);

How can we print the whole function ?
Please suggest me regarding this.

Thanks for your time

Cheers,
Ranga