If context or variable name doesn't make clear what a variable means, or is used for, one should add a comment. What would be your default value for a variable that's going to hold a reference to an object? Or a variable that could hold both a string and a numerical value? And if you assign 0, does that mean the variable will only hold integers? Only non-negative integers? Any numerical value? But even if you know, what insight to the program did it give to you?

I initialize variables only if it makes sense. I also tend to declare variables at their first use, which usually means the first time something is assigned to it - or it's done a line above the first assignment. Note that assigning default values will take time, although for larger blocks it won't matter too much:

#!/usr/bin/perl

use strict;
use warnings;

use Benchmark qw /cmpthese/;

cmpthese -5 => {
    init    => 'my $title = ""; my $page = 0; my $chapter = 0;
                $title && $page && $chapter',
    def     => 'my ($title, $page, $chapter);
                $title && $page && $chapter',
};

__END__
          Rate init  def
init 1165121/s   -- -38%
def  1876595/s  61%   --
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Abigail