Question

When should we use “use” and when “require” and when “AUTOLOAD” in perl ? I need a thumb rule for this.

Answer 1

use is equivalent to BEGIN { require Module; Module->import( LIST ); }

So, the main difference is that:

• Use is used at compile time

• Use automatically calls import subroutine (which can do anything but mostly used to export identifiers into caller’s namespace)

• use dies if the module can not be loaded (missing/compile error)

As such:

• When you need to load modules dynamically (for example, determine which module to load based on command line arguments), use require.

• In general, when you need to precisely control when a module is loaded, use require (use will load the module right after the preceding use or BEGIN block, at compile time).

• When you need to somehow bypass calling module’s import() subroutine, use require

• When you need to do something smart as far as handling load errors (missing module, module can’t compile), you can wrap the require into an eval { } statement, so the whole program doesn’t just die.

  You can simulate that with use but in rather in-elegant ways (trapping die signal in an early BEGIN block should work). But eval { require } is better.

• In all OTHER cases, use use

I didn’t cover AUTOLOAD as that’s a different beastie. Its usage is in cases when you want to intercpt calls to subroutines that you haven’t imported into your namespace.