That is because $obj is a Class::DBI::Iterator, not a User. If you want a User, you have to call search_like() in array context:

my ($obj) = User->search_like( 'upfirstname' => 'john%' );
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Now, normally you would use an array on the left hand side, since you likely have multiple matches. This is just a trick i like to use when i want only one Foo object and not an iterator.