It has to do with the special way that foreach works. In the statement foreach $n (1, 2, 3) {}, $n actually becomes the value in the LIST on each interation of the loop. It doesn't just get a copy of it in the variable $n. So in your example, $n would actually contain the read-only value 1 on the first iteration of the loop.

So what would happen if after the loop you tried to assign a value to $n? If it was not localized in the foreach loop, then you would be trying to assign a value to a read-only value, and you would get an error.

my $n;
foreach $n ( 1, 2, 3 ) { }
$n = 'foo';
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If $n was not localized, you would get a "Modification of a read-only value attempted ..." error in your code.

A better explaination of this issue can be found here in the perl5-porters archive.

- Cees