Depending on your specific requirements, use one of

• Proc::PID_File - takes care of PID file use for you.
• Proc::PID::File - OO way to do the above
• Linux::PID - for managing multi-threaded applications' PPID's on Linux (you still have to deal with files yourself).

CPAN is a wonderful thing, yes? There are also a number of pid server daemons that can track these types of things well, and Perl modules to talk to them. Or, you can solve the problem in your cron script with the insanely simple:

#!/bin/bash

if (ps ax |grep -q "my_perl_script_name") ; then exit; fi
my_perl_script_name
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