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>cat tst.pl 

...

open SELF, "< $0" or die ;
flock SELF, LOCK_EX | LOCK_NB  or die "$!";

>perl tst.pl 
A file descriptor does not refer to an open file. at tst.pl line 7.

.
.
...
389372: kfcntl(2, F_GETFL, 0x00000008)          = 1
389372: close(3)                    = 0
389372: kfcntl(2, F_GETFL, 0x00000008)          = 1

389372: fstatx(3, 0x30020848, 128, 010)         = 0
389372: kfcntl(3, F_SETFD, 0x00000001)          = 0