Many thanks to all who took the time to respond. Key seems to be using a writable lock file. The following version has been running over a month with no dups so I'll call it good.

# only one of us
my $LOCKFN = "/tmp/x.mylock";
open my $LF, '>', $LOCKFN or die "Can not create $LOCKFN: $!";
flock ($LF, LOCK_EX | LOCK_NB) or exit;
[download]