Perl Cookbook

Overriding Built-In Functions

Problem

You want to replace a standard, built-in function with your own version.

Solution

Import that function from another module into your own namespace.

Discussion

Many (but not all) of Perl’s built-in functions may be overridden. This is not something to be attempted lightly, but it is possible. You might do this, for example, if you are running on a platform that doesn’t support the function that you’d like to emulate. Or, you might want to add your own wrapper around the built-in.

Not all reserved words have the same status. Those that return a negative number in the C-language keyword() function in the toke.c file in your Perl source kit may be overridden. Keywords that cannot be overridden as of 5.004 are chop, defined, delete, do, dump, each, else, elsif, eval, exists, for, foreach, format, glob, goto, grep, if, keys, last, local, m, map, my, next, no, package, pop, pos, print, printf, prototype, push, q, qq, qw, qx, redo, return, s, scalar, shift, sort, splice, split, study, sub, tie, tied, tr, undef, unless, unshift, untie, until, use, while, and y. The rest can.

A standard Perl module that does this is Cwd, which can overload chdir. Others are the by-name versions of the functions returning lists: File::stat, Net::hostent, Net::netent, Net::protoent, Net::servent, Time::gmtime, Time::localtime, Time::tm, User::grent, and User::pwent. These modules all override built-in functions like stat or getpwnam to return an object that can be accessed ...