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#! perl -lw
use strict;
...

flock $fh, LOCK_EX;
flock $fh, LOCK_EX;

open my $fh, "<", "whatever" or die $!;
flock $fh, LOCK_EX;
...
die "open(): $!" unless defined $pid;
if ($pid > 0) { close $pip }
else          { flock($fh,LOCK_EX); print "got the lock"}

if ($pid > 0) { flock($fh,LOCK_EX); print "got the lock" }
else          { sleep(20) }