I often use:

use Fcntl qw !LOCK_EX LOCK_NB!;
die "Another instance is already running"
     unless flock DATA, LOCK_EX|LOCK_NB;

... your code here ...

# Don't forget __END__ or __DATA__
__END__
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Note that file locking solutions only prevent concurrent running on the same OS instance. If the job can be run from different machines, but you want to prevent concurrent runs from different boxes, you'll need a different solution. (Acquiring a database lock for instance).