Others have given some good explainations, no need to repeat them. But here are two tips: Data::Dumper is your friend, and you can use print statements to help tease apart these statements.

1. Data:Dumper is wonderful for figuring out data structures. Just add a

   use Data::Dumper;
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   at the start of your code, and then add a line like:

   print Dumper(\%students);
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   You'll get a nice printout of the data structure and its contents. By the way, the backslash returns a reference to the variable it precedes.
2. You can also try printing things to find out what they are. If you print a reference, you'll get something like SCALAR(0xda28) with a datatype and an address/uid.