The "my" is used to declare variables (see eibwen's post).

You should also include this:

  use strict;
  use warnings;
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at the top of your scripts -- what these will do is enforce (or warn) about certain things that will save you lots of trouble in the long-run, and point out compiling problems. Check out the manpages for each ("man strict", "man warnings") for more details. Something it is especially useful for is enforcing variable declarations:
This will run, but NOT do what you want:

  $myvariable = 3;
  print "Pi is exactly $myvaraible";
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This code, however, will give a very informative error message if you try to run it:

  use strict; 
  my $myvariable = 3;
  print "Pi is exactly $myvaraible";
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