Well - it wasn't too hard - here is one way to do it:

# Make sure there is not another copy of this script running
my $lockfile = '/var/run/proggie_check';
   open (LOCK, ">$lockfile") || die;    # No rights? - die
   flock(LOCK,2|4) || die;              # Lock it, no waitin' or die.
# --- We are the only copy around ---
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