i prefer to lock __DATA__, and i do it in an INIT block, like so:

## ensure only one instance is running
INIT { flock DATA => LOCK_EX | LOCK_NB or exit 1 }

## ...script...

__DATA__
[download]

i haven't run into a problem with this technique yet. (unless you call trying to save changes to the script while it's running a problem... ;)

~Particle *accelerates*