You should use the soft_close() or hard_close() methods. Please take a look at the documentation for the Expect module. Here's a simple example:
my $command = "cat > tmp.txt";
my $exp = Expect->spawn($command, @params)
or die "Cannot spawn $command: $!\n";
Or, if you really want to send Ctrl-C to your process, try this: $exp->send("\cC");. Or you can just send a signal to the process like Fletch recommends.