Programming Perl, 4th Edition

User-Defined Pragmas

Perl v5.10 added a way to easily create your own lexically scoped pragmata. The %^H hash contains information other code can inspect to get hints about what you’d like to do, and caller has a reference to the version of that hash in effect for the level you request:

my $hints = ( caller(1) )[10];

This is a simple hash with simple values. Without getting into the gory details, this hash may be shared between threads so the internals store it in a compact form that precludes any values other than integers, strings, and undef. That’s okay, because you really only need it to denote whether your pragma’s feature is on or off. This hash is also lexically scoped, so each lexical scope gets its own version.

To create your own pragma, define the three subroutines import, unimport, and in_effect. The first two are invoked implicitly by use and no. Typically, use turns on a feature by calling import, while no turns off that feature by calling unimport. Aside from any special processing you’d like, your import and unimport set a flag in %H. Outside your pragma, other code can call in_effect to find out whether your pragma is enabled, which you’ll handle by looking in %H for the value you set.

There are no rules on what you can put in %H, but remember that other pragmas also use this hash for their own work, so choose a key other code is unlikely to use, such as your package name.

Here’s a short pragma that replaces the built-in sqrt function with one that can handle negative ...