Class::Prototyped is probably a good option for this. It allows you to:

use Class::Prototyped;
my $p = Class::Prototyped->new();
$p->reflect->addSlot( sub1 => sub { print "this is sub1" } );
$p->sub1 # prints "this is sub1";
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It also provides a whole lot of other dynamic inheritance options if you want to build entire class hierarchies on the fly.