fields: use block eval instead of string eval

[mauke]

... and delete pointless require 5.005 as we already have use 5.008 three lines up.

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• This set of changes does not require a perldelta entry.

[jkeenan]

For the record, can you explain why we should use block eval rather than string eval here (and in some other p.r.s filed today)? Thanks.

[mauke]

String eval does two things: It compiles code at runtime, and it catches exceptions (both compile time and runtime errors). Block eval only does one thing: It catches exceptions.

Reasons why you might use string eval:

1. Metaprogramming, i.e. the code to run is not static, but assembled dynamically.
2. The code might not compile because it uses syntax only available in newer perl versions, so older perls would throw a parse error.
3. The code has compile-time effects (e.g. via use or BEGIN) that you would like to delay until runtime instead.

None of these apply here:

1. The code is a string constant.
2. Syntactically, there is nothing fancy going on. This code should compile in all perls back to 5.000.
3. It's all regular (runtime) code.

All we want to do is catch exceptions (specifically, test whether the equivalent of use warnings::register was able to run without errors). All else being equal, block eval is better because it more directly conveys the intent of the code, it is faster (no extra compiler invocation at runtime), and it uses less memory.