Simply put because while doesn't work like that :)

Explicitly, while doesn't iterate over the items of a list in the way that for does and it doesn't set $_ unless the expression is the diamond operator.

To get your example to work with while you will need to do something like :

    my @foo = sort (keys %data);

    while( $_ = shift @foo )
    {
       print "while: $data{$_}\n";
    }
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/J\