Well, as a rule of thumb: use my everywhere, and only if perl complains about, think about using local (e.g. with build in perl variables)

With local, you can also localize parts of datastructures, e.g.

use warnings; use strict;
use vars qw(@array);
@array = 1..20;
print "BEFORE: @array\n";
&TestSub();
print "AFTER: @array\n";
sub TestSub {
  local @array[4..10];
  @array[4..11] = 104..111;
  print "INSIDE: @array\n";
}
[download]

which outputs something like:

BEFORE: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
INSIDE: 1 2 3 4 104 105 106 107 108 109 110 111 13 14 15 16 17 18 19 2
+0
AFTER: 1 2 3 4 5 6 7 8 9 10 11 111 13 14 15 16 17 18 19 20
[download]

beware that 111 is not "localized" and so the change is global.