Are there any use cases where string eval is necessary in Perl?

Question

Can you provide any examples where using eval EXPR is really necessary? I'm asking because its generally discouraged.

Answer 1

Quite a few years ago, I wrote a static web site generator which used Perl (<-- interesting that my spell checker suggests "Peril" as a replacement for that) as its scripting language. It allowed the user to enter information about different levels of a hierarchy - the first to market was for a cemetery where you had clients (paying customers) who owned memorials (for the deceased, obviously).

It had a GUI where you entered variables such as (tremble at my graphical skills here):

+------------+----------------------------+
| ClientName | Pax Diablo                 |
+------------+----------------------------+
| Address    | 666 Hades St, Washinton DC |
+------------+----------------------------+

and picture variables as well.

In addition, for each market (such as the afore-mentioned cemetery), there was a controlling script which basically ran and processed each record at multiple hierarchy levels to generate the static content. Before then, the program had turned all those variables into things like:

$Var{"ClientName"} = "Pax Diablo";

(I think that syntax is right, I haven't done any Perl development for a while). The end result was a full web site with lots of clients, lots of memorials, search pages and a nice little money earner while it lasted (I was blown out of the market by funeral directors providing the same thing as a 'free' service within their regular packages).

But the whole thing was basically written in minimal Perl which only did two things:

• turned the variables into Perl assignment statements; and
• ran the market-specific Perl code.

Both the execution of the assignment statements and the market-specific Perl code was done with eval.

So it's not as useless as you may think. It was a cheap way to introduce scripting into an application.

Answer 2

String eval is the only way to make new code out of strings. This is rarely necessary, but there are a few valid use cases, for example when converting templates to runnable code. Other than that, there are very few valid uses for it. eval "require $module_name" is a well known idiom, but using for example Module::Load is a better idea than that IMO.

Answer 3

String eval is the only way to:

1. execute new code in a runtime decided package
2. add overloading to a package at runtime (at least according to overload.pm)
3. execute arbitrary strings of Perl
4. compile macro type substitutions

String eval should not be used to:

1. evaluate symbolic references
2. execute any code from the user
3. trap errors
4. if there is any other way to do it

eval can also be used to create ad-hoc communication protocols between different systems or different languages that also have eval (provided that everything is secure of course. JSON can be seen as a more secure, but limited subset of this method)