Re: The 'eval "require $module; 1"' idiom

[davido]

This is a generalized idiom, in the form of:

eval "something_that_might_fail(); 1;" or die "Message.";

In the general case, it makes sense because "something_that_might_fail()" could be some expression where "false" or "undef" are not out of band for success (ie, are perfectly valid return values even for a successful call).

In the specific case of "eval "require Some::Module; 1;" or die "Message.", your "Case 3" and "Case 3b" observations are accurate; require will return 1 on success. In this specific case, the 1; is redundant, though semantically it does maintain eval's return value to not be tied to the return value of the primary "thing that might fail."

On the one hand, one might say that "eval "require Foo; 1;" or die "Message";" is cargo culted. On the other hand, the idiom as a general tool is useful, and probably doesn't need to be optimized further just because this specific application doesn't need the 1;. It's healthy to decouple the thing that might fail's return value from the success sentinel, which is what the idiom does.

By the way; this is a very well researched question. Kudos.

[sedusedan]

Thanks for pointing out about the general term. Although the majority of the general form is for require/use, people do use it for other purposes too:

http://grep.cpan.me/?q=if+%5C%28eval+%22.%2B%3F%3B+1%22

[davido]

I wish the grep.cpan.me supported lookaheads and lookbehinds, which would allow me to search a little more specifically for situations where the idiom is used outside of the "require" and "use" use case. If it did, I might try something like:

if \(eval ["'](?!require|use).+?;\s*1;?["']

[DouglasDD]

I wish the grep.cpan.me supported lookaheads and lookbehinds

We can use [^ur] as a cheap trick to grep for cases other than use or require:
(if|unless) \(eval +["'][^ru].+?; 1;?["']