You can find full documentation on chomp in the Perl documentation. But to answer your specific questions:

1. The value is the total number of characters removed over all list elements passed. If you chomp a scalar, the return value will generally be 0 or 1.
2. It returns this value. You can see this from the code:

   use strict;
   use warnings;
   
   my @input = <DATA>; # Take in the entire file
   
   my $count = chomp @input;
   print "Characters removed: $count\n";
   
   
   __DATA__
   This is some text   
   Each line has a newline   
   That makes 3 total
   [download]

   In general, questions like this can be answered quickly by just creating a mock-up script and playing around.

3. See point 2.

As side notes, since Perl is case sensitive, CHOMP means something different than chomp, though your intent is obvious enough. As well, note chomp is a function (perlfunc), not an operator (perlop). In particular, chomp is a function with side-effects - it changes the variables you pass it. The difference may explain why you were searching for some special Perl variable (perlvar).