How can I read piped input in Perl on Windows?

I am trying to create something in Perl that is basically like the Unix tee command. I'm trying to read each line of STDIN, run a substitution on it, and print it. (And eventually, also print it to a file.) This works if I'm using console input, but if I try to pipe input to the command it doesn't do anything. Here's a simple example:

print "about to loop\n";
while(<STDIN>)
{
  s/2010/2009/;
  print;
}
print "done!\n";

I try to pipe the dir command to it like this:

C:\perltest>dir | mytee.pl
about to loop
done!

Why is it not seeing the piped input? (I'm using Perl 5.10.0 on WinXP, if that is relevant.)