Okay. Try adding $^E to your logging; it might make for a clearer picture.

It would be easier to suggest something if you posted the thread code, but at a very minimum I'd rewrite that open something like this:

my $retries = 5;
my $fhLock;
while( $retries and not open $fhLock, ">", $lockfile ) {
    warn "Failed to open $lockfile: $! [$^E]";
    sleep 1; 
    --$retries;
}
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And then wait until it happens again. That should tell you whether its a temporary, transitory problem or not; and perhaps shed more light on the cause.