Because

say $instance;
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is nicer than

say $instance->as_string;
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That's it. Because it's nice.

And it's not like it takes anything away. When's the last time you saw Class=HASH(0xDEADBEEF) and thought it was useful?^[1]

But it's best to give the choice, though.

use overload '""' => \&as_string;

sub as_string {
   my $self = shift;
   ...
}
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1. The overload module provides a function to get that stringification if you really do need it. Also, the referenced var type, its address and the class associated with it are all available through Scalar::Util functions.