I've been reading a lot about constructing interpreters and VMs lately, and one thing that seems common to most new ones is the seemingly obligatory inclusion of closures; and the almost equally obligatory tales of woe, sleepless nights and extreme programming hardship that are required in order to implement them in an efficient and elegant manner.

Then the thought that formed in my mind was that whilst I am quite happy to make extensive use of them, I was also quite happily solving my programming problems in languages that don't support them for 15+ years before I became acquainted with them.

So then the questions that came to me were: a) how much use do people make of them in Perl? b) How many of those uses are actually 'killer applications' of closures, rather than trivial or convenient uses of up-values in anonymous subroutines that could just as easily be done using some other mechanism that doesn't impose such demands upon the VM?

One of the 'killer applications' of closures often cited in FP circles, is their links to continuation passing. But continuation passing is just one method of solving a problem that can be solved in various other ways. And most of those other ways seem to impose far less runtime burdens than CP whilst achieving the same goals.

Hence my very open question to try and get a feel for what people do with closures in real-world code.