Yes it does appear that you have a race condition in that code. Since what you are doing is very similar to the needs of a read/write operation I'd suggest using a second lock file (called a semaphore file) to lock the whole operation between testing the existance of PID_LOCK and finally writing $$ to the file. Eg.

sub single_instance{
    $SIG{INT} = sub {exit()};
    my ($prog) = $0 =~ m|(?:.*[/\\])?(.*)$|;
    my $tmp_dir = $^O =~ "MSWin32" ? "C:/Windows/Temp" : "/tmp";
    my $lockfile = "$tmp_dir/$prog.pid"; 
    local *PID_FILE; 
    open(SLOCK,">$tmp_dir/$prog.slock") or die("single_instance(): Can
+'t open slock: $!\n");
    flock(SLOCK,2);
    if (-e $lockfile){ 
        open(PID_FILE, "<$lockfile") or die("single_instance(): Can't 
+open $lockfile: $!\n"); 
        my $running_pid = <PID_FILE>; 
        close(PID_FILE); 
        chomp($running_pid); 
        if (kill 0, $running_pid){ 
            die("Already running as pid $running_pid\n"); 
            } 
        } 
    open(PID_FILE, "+>$lockfile") or die("single_instance(): Can't ope
+n $lockfile: $!\n"); 
    print PID_FILE "$$\n"; 
    close(PID_FILE); 
    close(SLOCK);
    chmod(0666, $lockfile);
    eval "END {unlink '$lockfile'}"; 
    die("single_instance(): $@\n") if($@); 
    }
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This avoids the race condition because whichever process that first gets exclusive lock on the semephore file is able to read and write to the lock file before the other process can even test for the lock file's existance.

-caedes