As everyone else has asked, what number would you propose it be treated as?

Perl doesn't have a single equality operator, it has two which operate on specific types of values:

• == / != for numeric values
• eq / ne for string values

A numeric value gets promoted to its string representation when used as a string. A string value gets converted to a numeric value (if possible; 0 if it's not a valid numeric representation) when used as a number.

So when you consider what could be done with the "wrong" kind of value in a given context there's two possibilities:

• treat it as a domain error (you're not allowed to compare these two types of values; see my Ruby example)
• convert it to a value in the correct domain (which is what Perl does)

Given that the second choice was made, again what other value would you expect? I might could see an argument being made for NaN, but the choice of 0 makes lots of things "just work" nicely (especially for things like quick 1-liners or 5 minute throw-away scripts where you really aren't that concerned with data validation).

I certainly understand operator based context and all that stuff that makes Perl so Special (well, obviously not all of it). I guess I just never really thought about what numerical value undef should have. My understanding of undefined is that it shouldn't have any value at all, by definition. I suppose that something like NaN would be closest to my expectations.