new at Perl myself but I couldn't see that anyone had answered your query "what is $_?".

$_ is one of Perl's built-in variables - it is the default variable (i.e. what's currently being input or matched) and frequently if not explicitly set it will be assumed.

These pairs are all the same: (these examples are from perlvar (http://perldoc.perl.org/perlvar.html)

$_

The default input and pattern-searching space. The following pairs are
+ equivalent:

    while (<>) {...}    # equivalent only in while!
    while (defined($_ = <>)) {...}

    /^Subject:/
    $_ =~ /^Subject:/

    tr/a-z/A-Z/
    $_ =~ tr/a-z/A-Z/

    chomp
    chomp($_)
[download]

It's a very important variable to get your head around so do ask in the Chatterbox if you're unsure. (at least that's what I would do).