Here's a diff. This is a vanilla 5.8.3 on Linux PPC.

--- postfix     2004-07-12 22:45:53.000000000 -0700
+++ block       2004-07-12 22:45:44.000000000 -0700
@@ -3,23 +3,23 @@
 2     <;> nextstate(main 1 -e:1) v ->3
 5     <2> sassign vKS/2 ->6
 3        <$> const(IV 0) s ->4
-4        <0> padsv[$total:1,2] sRM*/LVINTRO ->5
-6     <;> nextstate(main 2 -e:1) v ->7
-7     <;> nextstate(main 2 -e:1) v ->8
+4        <0> padsv[$total:1,4] sRM*/LVINTRO ->5
+6     <;> nextstate(main 3 -e:1) v ->7
 j     <2> leaveloop vK/2 ->k
-c        <{> enteriter(next->g last->j redo->d) lKS ->h
--           <0> ex-pushmark s ->8
--           <1> ex-list lK ->b
-8              <0> pushmark s ->9
-9              <$> const(IV 1) s ->a
-a              <$> const(NV 1000000) s ->b
-b           <$> gv(*_) s ->c
+b        <{> enteriter(next->g last->j redo->c) lKS ->h
+-           <0> ex-pushmark s ->7
+-           <1> ex-list lK ->a
+7              <0> pushmark s ->8
+8              <$> const(IV 1) s ->9
+9              <$> const(NV 1000000) s ->a
+a           <$> gv(*_) s ->b
 -        <1> null vK/1 ->j
-i           <|> and(other->d) vK/1 ->j
+i           <|> and(other->c) vK/1 ->j
 h              <0> iter s ->i
 -              <@> lineseq vK ->-
-f                 <2> add[t2] vKS/2 ->g
-d                    <0> padsv[$total:1,2] sRM ->e
+c                 <;> nextstate(main 2 -e:1) v ->d
+f                 <2> add[t4] vKS/2 ->g
+d                    <0> padsv[$total:1,4] sRM ->e
 -                    <1> ex-rv2sv sK/1 ->f
 e                       <$> gvsv(*_) s ->f
 g                 <0> unstack v ->h
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Thanks, somehow I missed that first nextstate. It's still there in bleadperl, and seems totally unnecessary to me.

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.

nextstate preps things between statements. It sets a pointer used to give warn and die messages a line number, clears the tainted-expression flag, resets the stack pointer, and frees any temporary variables generated by the previous statement.

scoping is done mostly by scope (for a scope containing a single statement), or enter* & leave (for a scope containing multiple statements).