Okay, so it works as expected:

Test code:

#!/usr/bin/perl
use strict;
use warnings;
use Fcntl qw(:flock);

open our $lockfile, '<', $0 || die $!;
if ( (1 == 1) || (!flock $lockfile, LOCK_EX | LOCK_NB) ) {
  print "in loop\n";
  sleep 30;
  if (flock $lockfile, LOCK_EX | LOCK_NB) {
    print "locked\n";
  } else {
    print "not locked\n";
  }
  sleep 30;
}
[download]

First instance:

in loop
locked
[download]

Second instance:

in loop
not locked
[download]