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;

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In reply to Re: singleton lock not reliable by pidloop
in thread singleton lock not reliable by pidloop

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