How should I use the "my" keyword in Perl?

I keep seeing the "my" keyword in front of variable names in example Perl scripts online but I have no idea what it means. I tried reading the manual pages and other sites online but I'm having difficulty discerning what it is for given the difference between how I see it used and the manual.

For example, its used to get the length of the array in this post: Find size of an array in Perl

But the manual says:

A my declares the listed variables to be local (lexically) to the enclosing block, file, or eval. If more than one value is listed, the list must be placed in parentheses.

What does it do and how is it used?

my restricts the scope of a variable. The scope of a variable is where it can be seen. Reducing a variable's scope to where the variable is needed is a fundamental aspect of good programming. It makes the code more readable and less error-prone, and results in a slew of derived benefits.

If you don't declare a variable using my, a global variable will be created instead. This is to be avoided. Using use strict; tells Perl you want to be prevented from implicitly creating global variables, which is why you should always use use strict; (and use warnings;) in your programs.

Related reading: Why use use strict; and use warnings;?

Quick summary: my creates a new variable, local temporarily amends the value of a variable

In the example below, $::a refers to $a in the 'global' namespace.

$a = 3.14159;
{
  my $a = 3;
  print "In block, \$a = $a\n";
  print "In block, \$::a = $::a\n";
}
print "Outside block, \$a = $a\n";
print "Outside block, \$::a = $::a\n";

# This outputs
In block, $a = 3
In block, $::a = 3.14159
Outside block, $a = 3.14159
Outside block, $::a = 3.14159

ie, local temporarily changes the value of the variable, but only within the scope it exists in.

Source: http://www.perlmonks.org/?node_id=94007

Update

About difference between our and my please see

• What is the difference between my and our in Perl?

(Thanks to ThisSuitIsBlackNot).

Private Variables via my() is the primary documentation for my.

In the array size example you mention, it's not used to find the size of the array. It's used to create a new variable to hold the size of the array.