Here's another way.

my $obj = Class->new();
my $obj2 = Class->new(string => 'foo', num => 11);

package Class;
sub new{
    my $class = shift;
    my $attr = { @_ };
        bless $attr, $class;
}
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UPDATE (UTC 0800):

Of course, this doesn't really solve your problem -- you pass a hash reference as "prototype" when creating your object, I pass the hash itself. My "solution" would change your interface; this might not be possible for you.