Couldn't find the FM either. Here's the code...I guess this is RTFC ;) It's in Pod::Perldoc. Command line option -X translates to the attribute podidx in the Pod::Perldoc::options_processing method. Pod::Perldoc::grand_search_init is where packages are found matching the search package.

sub grand_search_init {
    my($self, $pages, @found) = @_;

    foreach (@$pages) {
        if ($self->{'podidx'} && open(PODIDX, $self->{'podidx'})) {
            my $searchfor = catfile split '::', $_;
            $self->aside( "Searching for '$searchfor' in $self->{'podi
+dx'}\n" );
            local $_;
            while (<PODIDX>) {
                chomp;
                push(@found, $_) if m,/$searchfor(?:\.(?:pod|pm))?\z,i
+;
            }
            close(PODIDX)            or die "Can't close $$self{'podid
+x'}: $!";
            next;
        }
        ...
[download]

It looks like the .idx file would speed up searches for package pod if the .idx file existed.

Hope that helps.