Thank you! I dropped the .lock file and am now locking the script itself. It appears I can lock it again, with no apparent side effect, if it was locked earlier. Found a neat way of referencing the running script without using DATA but now can't find the site to reference it.

The lock_retrieve and lock_nstore were intended to keep other processes waiting, whilst the updated data is being flushed to the cache file.

Herewith the resulting script snippet:

my $cache_ttl = 60;
my $cachefile = '/tmp/mtapi.'.$router.'.cache';
my %bgp_peer;
my $cached = 0;
my $modified = (stat($cachefile))[9];
open our $lockfile, '<', $0 || die $!;
if (defined $modified) {
  if (($modified >= (time-$cache_ttl)) || (!flock $lockfile, LOCK_EX |
+ LOCK_NB)) {
    if (%bgp_peer = %{lock_retrieve($cachefile)}) { $cached = 1 };
  }
}
if ($cached == 0) {
  if (!flock $lockfile, LOCK_EX | LOCK_NB) { print STDERR "Another pro
+cess is already using MikroTik API.\n"; exit 6 };
  $api->connect();
  $api->login();
  %bgp_peer = $api->get_by_key('/routing/bgp/peer/print', 'name');
  $api->logout();
  lock_nstore \%bgp_peer, $cachefile;
  chmod 0660, $cachefile;
  flock $lockfile, LOCK_UN;
}
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Could someone please confirm that Perl would skip trying to acquire a lock when the first 'if' condition is true?

Should I rather place the test in to a nested condition, completely outside of the first?

if ( (1 == 1) || (!flock $lockfile, LOCK_EX | LOCK_NB) )

PS: Things appear to be working as expected, but I would still like to know, for my own peace of mind.

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;
}
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First instance:

in loop
locked
[download]

Second instance:

in loop
not locked
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