I believe (but do not know for absolutely sure) that nextstate is the opcode that sets things up to enter a block or maybe it is exit the block, I can't remember for sure (something is making me think that the op-tree is traversed/processed in post-order rather than pre-order and that might make the difference). If you are really interested you might want to check out Simon Cozens Perl 5 Internals paper, in particular the end part talks about some of the output of the B modules, and how to read/understand them.

The (overly simplistic) reason why entering an exiting a block has a performance penalty is that perl is lexically scoped. So when entering each block a new lexical scratchpad needs to be set up, and then upon exiting the block all variables need to be restored.