Programming Perl, 4th Edition

do (file)

do FILE

The do FILE form uses the value of FILE as a filename and executes the contents of the file as a Perl script. Its primary use is (or rather was) to include subroutines from a Perl subroutine library, so that:

do "stat.pl";

is rather like:

scalar eval `cat stat.pl`;   # `type stat.pl` on Windows

except that do is more efficient, more concise, keeps track of the current filename for error messages, searches the directories listed in the @INC array, and updates %INC if the file is found. (See Chapter 25.) It also differs in that code evaluated with do FILE cannot see lexicals in the enclosing scope, whereas code in eval FILE does. It’s the same, however, in that it reparses the file every time you call it—so you might not want to do this inside a loop unless the filename itself changes at each loop iteration.

If do can’t read the file, it returns undef and sets $! to the error. If do can read the file but can’t compile it, it returns undef and sets an error message in $@. If the file is successfully compiled, do returns the value of the last expression evaluated.

Inclusion of library modules (which have a mandatory .pm suffix) is better done with the use and require operators, ...