perlvar

$FORMAT_LINES_LEFT
$-
The number of lines left on the page of the currently selected output channel. Used with formats. (Mnemonic: lines_on_page - lines_printed.)

Perl has lots of these special variables. They're independent of each other (though some have related areas of utility.) In the general case, arbitrarily substituting one for another is the wrong thing, and you should expect it to break your code.