Otherwise, demands that a library file be included if it hasn't already been included. The file is included via the do-FILE mechanism, which is essentially just a variety of eval(). Has semantics similar to the following subroutine:

    sub require {
        local($filename) = @_;
        return 1 if $INC{$filename};
        local($realfilename,$result);
        ITER: {
            foreach $prefix (@INC) {
                $realfilename = "$prefix/$filename";
                if (-f $realfilename) {
                    $result = do $realfilename;
                    last ITER;
                }
            }
            die "Can't find $filename in \@INC";
        }
        die $@ if $@;
        die "$filename did not return true value" unless $result;
        $INC{$filename} = $realfilename;
        $result;
    }

Note that the file will not be included twice under the same specified name. The file must return TRUE as the last statement 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. But it's better just to put the `` 1;'', in case you add more statements.

If EXPR is a bareword, the require assumes a ``.pm'' extension and replaces ``::'' with ``/'' in the filename for you, to make it easy to load standard modules. This form of loading of modules does not risk altering your namespace.

For a yet-more-powerful import facility, see use and the perlmod manpage.