Actually, ( sort keys %hash ) creates a list, not an anonymous array (see sort and keys). Anonymous arrays are created using square brackets (see perlref). Please see (tye)Re: What is the difference between a list and an array? for a nice explanation of the difference between lists and arrays (thanks, tye).

Secondly, $key is an alias to each value in the list, which means modifying $key will modify the original value (see perlsyn), as the following example shows:

use strict;
use warnings;

my @array = ( 1 .. 5 );

foreach my $value ( @array )
{
    $value--;
}

print join( ', ', @array ), "\n"; # 0, 1, 2, 3, 4
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HTH

Update: added links for sort, keys, and (tye)Re: What is the difference between a list and an array?.

Quote:Secondly, $key is an alias to each value in the list, which means modifying $key will modify the original value :EndQuote

It will change the value in the list returned by keys and sort, but not in the original hash, e.g.

my %hash = (1 => 'a', 2 => 'b', 3 => 'c');

foreach my $value (sort keys %hash )
{
    $value--;
}

print join( ', ', sort keys %hash ), "\n"; # 1, 2, 3
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-imran