This is highly educational for me, thanks very much for posting it. I must admit that I use $_ very sparingly, partly because I don't understand such things very well. I think it's also because of my longstanding habit of being very explicit (C, need I say more?). I'm still getting used to lines like

while (<>) {
  chomp;
  print $_, "\n";
}
[download]

because my old habits say "Where did $_ come from???" Going through such exercises as the above is helping me learn better, more Perlish ways.
I think it's fun and worthwhile to play with such things. And I'm not surprised at being confused on a Monday morning.