I wrote Script::Singleton exactly for this task.

It uses the shared memory functionality of IPC::Shareable which I also maintain.

To use it, you literally only need to use it:

use Script::Singleton;
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...done.

It uses the script's path and filename as the "glue" aka. shared memory key for identification. If you want to use a custom glue:

use Script::Singleton glue => 'UNIQUE GLUE STRING';
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Best part I like about it is that if there's a system interruption or reboot, you don't have to worry about lock files hanging around.

Here's an example (using the module with the warn parameter which is false by default):

use warnings;
use strict;

use Script::Singleton warn => 1;

sleep 5;
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Run the script in one CLI, and within five seconds, run it in a second window and you'll get: