Re your update, here's a version that doesn't use use vars:

#!/usr/bin/perl -w
use strict;
our $x = "123 56"; 
$x =~ / /g;
print "$x, ", pos($x), "\n";
{ local $x }
print "$x, ", pos($x), "\n";
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I think you meant, "another reason to avoid using package variables".

By the way, I don't see how you got it to work with strict on. It doesn't work if you keep $x as a package variable, and local $x doesn't work if you changed $x into a lexical (i.e. used my $x).