my is probably what you want, since it creates a lexical variable that can't be tampered with outside of the block its delcared in. Take a look at this code:

#!/usr/bin/perl -wT
use strict;

# use an our-variable....
sub count_our {
   our $num ||= 0;
   $num++;
}
sub print_our {
   our $num ||= 0;
   print $num, "\n";
}

# use a my-variable....
{
 my $num = 0;
 sub count_my {
    $num++;
 }
 sub print_my {
    print $num, "\n";
 }
}

# thousand lines of code here

# $num is common enough that we use it somewhere else as well
my  $num = 10;
our $num = 10;

# thousand lines of code here

# did we mess up the 'my' variable?
count_my();
print "My count:  ";
print_my();

# how about the 'our' variable?
count_our();
print "Our count: ";
print_our();
[download]

Notice that the our-variable gets fubared somewhere in the middle of this script. The my-variable is truly encapsulated and can't be modified except by count_my() and print_my().

-Blake