I don't think Foreach Loops support multiple iterator variables like that. Take a look at this B::Deparse output (tip #6 from Basic debugging checklist):

$ perl -MO=Deparse 901847.pl
use warnings;
use strict 'refs';
{
    my(%hash) = ('one', 1, 'two', 2);
    foreach $_ (my($key, $value) = each %hash) {
        print $key, "\n";
    }
}
[download]

It shows that the loop is really using $_ for the iterator.

Update: Now I have changed the OP code a little, just to print more info:

use warnings;
use strict;

my %hash = ("one" => 1, "two" => 2, );
foreach ((my $key, my $value) = each %hash) {
    print "\$key=$key \$value=$value \$_=$_\n";
}

__END__

$key=one $value=1 $_=one
$key=one $value=1 $_=1
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It seems like each is evaluated once in list context. It happens to choose the 'one' key and stores that in $key and stores the value '1' in $value. It never evaluates each again. Then each time through the loop, each item of the list ('one', '1') is assigned to $_.