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my $type = ref $first;
...
croak("...")
  unless $type eq 'HASH' or (ref($first) and UNIVERSAL::isa($first, 'H
+ASH'))

use overload (); # added (), according to ysth's suggestion
use Scalar::Util qw/blessed reftype/;
...
            and overload::Method($thing, $overload{$type});
    return 0;
}