Another possibility is that you use some files in you script, and that you use them like this:

open FILE,"my_data" or die "Error\n";
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or

system("exec.pl");
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But you must be careful: you don't know what is the current directory of cron when it executes your script. So you must precise the entire path for files you use.
e.g

open FILE,"/my/path/my_data" or die "Error\n";
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or

system("/my/path/exec.pl");
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Hope this helps

Zejames