Of course, shortly after posting - I found the answer, in this answer

use strict;
use Fcntl qw(SEEK_SET F_WRLCK F_UNLCK F_SETLKW);

my($pack);


open(FILE,">a");
$pack = pack('s s l l s', F_WRLCK, SEEK_SET, 0, 1, 0);
print(fcntl(FILE, F_SETLKW, $pack) . "\n");
sleep(20);
$pack = pack('s s l l s', F_UNLCK, SEEK_SET, 0, 1, 0);
print(fcntl(FILE, F_SETLKW, $pack) . "\n");
close(FILE);
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This worked for me on my Linux implementation, to test whether locking over NFS works.