# Hack for sketching
use lib '/home/bsb/sb/lib';
BEGIN {
    # look in cwd first
    unshift @INC =>
        sub {
            $_[1] =~ m!^SD/(?:.*/)?(.*)! || return;
            #print STDERR "Loading $_[1] $1\n";
            open my($f), $1;
            #$INC{$_[1]} = "./$1";
            $f;
        };
}
[download]

Brad

One possible use would be to interpose a PerlIO::via layer or other translation mechanism in reading module files. For instance, suppose we think we can save disk IO time by storing gzipped versions of our modules. A sub to read and unzip the file might be written like this (completely untested):

use Compress::Zlib qw/gzopen gzeof gzread gzclose gzerror/;
unshift @INC, sub {
    my ($cref, $filename) = @_;
    $filename = "/path/to/compressed/libs/${filename}.gz";
    return unless -f $filename;
    my $text;
    {
        my $gz = gzopen( $filename, 'rb')
            or warn $gz->gzerror() and return;
        while ( !$gz->gzeof() ) {
            my $buffer;
            my $bytes = $gz->gzread($buffer);
            warn $gz->gzerror() and return
                if $bytes == -1;
            $text .= $buffer;
        }
        $gz->gzclose();
    }
    require 5.008;
    open my $fh, '<', \$text or return;
    $fh;
}
[download]

Other uses come to mind. The sub could check md5sum for the requested module, returning undefined for good ones and dieing for bad. A source filter could be waved over the module. Decryption could be done much like the gzip example.

It might be fun to generate modules on the fly with Template Toolkit.

You have the gist of it correct. Acme::Inc provides a (counterproductive) example of use.

If you really want to see some module that uses that, take a look in PAR

it says

                                   If there is no filehandle
  (previous item), then this subroutine is expected to
  generate one line of source code per call, writing the line
  into $_ and returning 1, then returning 0 at "end of file".
[download]

but this is never the case, passing undef doesn't help.

The "previous item" always has to be a filehandle.

Otherwise the generator-callback "this subroutine" is never called.

Passing a dummy filehandle helps.

update

DARN! looking at this example Re: Cancel/no-op a require reveals the problem with the documentation.

In the hook you can either return FILEHANDLE, CODEREF; or return CODEREF; but the docs imply return undef, CODEREF; for the second case.