my is definitly faster than local,
according to Advanced Perl programming and the Camel Book, the real gurus will explain why in details...
(tilly, merlyn, anyone else ???)

But in short (I don't master the topic yet ;-) 'my variables' uses 'scratchpad' (a special table assigned to a scope),
rather than the usual typeglobs table (and they don't use the assign/reassign value mechanism used by local).

The gain is double :
You access you variable directly through the scratchpad ($a)->(the address of $a)
rather than via the typeglob table (a)->(the typeglob table)->(the address of $a in this typeglob table)
You avoid the the saving/restoring of the previous value of the local variable when you enter/leave the scope.

"Trying to be a SMART lamer" (thanx to Merlyn ;-)