Well it's like this. In YAML, every document has a "schema" which determines exactly what type or tag every node in the document has. For two applications to share a document, they need to agree on a "schema".

I quote "schema" because it isn't really a tangible thing like XML schema language. In YAML, there is not yet an official schema language.

A YAML schema can come in one of the following forms:

• Every scalar is a string. This is (or at least should be) the default schema for your general purpose YAML Dumper/Loader.
• YAML implicit typing is honored. (Implicit typing is what turns 07102 into 3650) This is usually not the default, but some processors might choose this.
• The YAML Loader/Dumper chooses a schema of its own. This is in effect what all loader/dumpers should do. Basically it means that it can load whatever it dumped into the same internal structure.
• The application chooses the schema. This is used when every scalar is a string, usually. The program says, "oh the string at $y->{event}{date} should be a date object".
• An external schema document can be used to declare the type of each node. Nobody does this yet.

YAML.pm's strategy is to load all unquoted scalars as strings except for ~ which becomes undef.

If you want to use YAML.pm with other yaml processors you need to agree on a schema, which really means you need to find out what they do and make sure everything is in sync for your needs.

Cheers, Ingy