$^I

I just looked it up on perldoc perlvar, but I am still not sure what it 
does.



 $^I     The current value of the inplace-edit extension.  Use "undef" 
to disable inplace editing.  (Mnemonic: value of -i
               switch.)

I was reading perl cookbook and saw this example, and was wondering what 
that is....


if (@ARGV) {
    $^I = ".orig";
} else {
     warn "$0: Reading from stdin\n" if -t STDIN;
}