#!/usr/local/bin/perl
use strict; use warnings; $| = 1;

use Fcntl qw( :flock);

my $name = 'ttt'; # process name

my $pid_file = "/var/run/$name.pid";

open my $pid_handle, '+<', $pid_file or die "Can't access '$pid_file': $!";
flock $pid_handle, LOCK_EX | LOCK_NB or die "Process $name is still active";

truncate $pid_handle, 0 or die "Truncating '$pid_file' failed: $!";
print $pid_handle "$$\n";

# Normal processing starts here
#
# ...

sleep 10; # for testing