Programming Perl, 4th Edition

require

require VERSION
require EXPR
require

This function asserts a dependency of some kind on its argument.

If the argument is a string, require loads and executes the Perl code found in the separate file whose name is given by the string. This is similar to using a do on a file, except that require checks to see whether the library file has been loaded already and raises an exception if any difficulties are encountered. (It can thus be used to express file dependencies without worrying about duplicate compilation.) Like its cousins do and use, require knows how to search the include path stored in the @INC array and to update %INC on success. See Chapter 25.

The file must return true as the last value to indicate successful execution of any initialization code, so it’s customary to end such a file with 1 unless you’re sure it’ll return true otherwise. (This requirement may be relaxed in the future.)

If require’s argument is a version number of the form v5.6.2, require demands that the currently executing version of Perl be at least that version. (Perl also accepts a floating-point number ...