Considering flocking the data handle only takes a single line, there isn't much to gain from using a module (the module does more, but the OP doesn't need that). After ending the script with __DATA__ (or __END__), just add: 
flock DATA, 6 or die "Duplicate instance";

[download]
Or use the Fnctl module and use LOCK_EX|LOCK_NB instead of 6.

In reply to Re^2: how to exit if tool is already running by JavaFan
in thread how to exit if tool is already running by srikrishnan

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