There are many ways to ensure single instance of perl program is running. PID files are the traditional way to do it. we can also hold a lock on a file,for example the program itself. This small piece of code will do the trick:

use Fcntl ':flock';
open my $self, '<', $0 or die "Couldn't open self: $!";
flock $self, LOCK_EX | LOCK_NB or croak "This script is already runnin
+g";
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One advantage over PID files is that files automatically get unlocked when the program exits. It is much easier to implement in a reliable way.