From perldoc IO::Select (for the select method):

The result will be an array of 3 elements, each a reference to an array which will hold the handles that are ready for reading, writing and have exceptions respectively.

See also: Writeup Formatting Tips

but then why is there a foreach? What is that loop actually doing to the sets variable?

Though select returns three elements, your code is only using the first one, $sets, a reference to an array of handles ready for reading. To de-reference this array reference $sets, you use @$sets, as described in the perlreftut link already provided by toolic. Note that @$sets is equivalent to @{$sets} (the form more commonly used in perlreftut). You need to take the time to read perlreftut and to understand how references work in Perl.

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