The script you're looking at is probably using the Getopt::Std module, which makes it easy to pass in parameters as flags from the command line. For example, a program called like this:

./grobilize.pl -t 20 -n 44
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will end up containing the variables $opt_t and $opt_n, set to the appropriate value. Like the earlier posts have explained, if you have use strict on (as you should), you need to pre-declare those as globals to avoid getting a compile-time error.