A few things you can do to make it easier to learn what's going on with your code. At the top of your file, along with other pragmas include:

• use diagnostics -v;
  use Data::Dumper;
  
  ### YOUR CODE ###
  
  print Dumper(@$sets);
  [download]

  • The first line will give you more debugging information on any errors/warnings your code is returning.
  • The second (use Data::Dumper) will reveal the contents of variables such as your @$sets, using print Dumper(@$sets);

References in Perl can be hard to learn (as can navigating Perl's documentation) but it's important you do so in order to not struggle with the language. Good luck!

See also:
• PerlMonks: How can I visualize my complex data structure?
• Elements of Programming with Perl Ch. 4 Data : Types and Variables
• Effective Perl Programming Ch. 2 item 20 and Ch. 6 References
• PerlMonks: Getting Better at Finding Answers
• PerlMonks: Perl documentation documentation