What's the underlying model here?

You have to consider the full CPAN object model. Ultimately, actions all fall to a CPAN::Distribution object and "force" is just a method that sets a flag and invokes other methods. The shell is a CPAN::Shell object that converts your commands to actions upon the appropriate objects.

For example, installing a module means creating a module object, finding the distribution object that corresponds to it, and then installing the distribution. (Well, that's the short version anyway.)

Here's a modified excerpt of what I use in Perl::Dist as part of building Vanilla Perl. (Warning: Perl::Dist as a whole sucks and needs to be rewritten; but using snippets is OK.)

print "Preparing to install $name from CPAN\n";

$obj = CPAN::Shell->expandany( $name ) 
    or die "CPAN.pm couldn't locate $name";

if ( $obj->uptodate ) {
    print "$name is up to date\n";
    exit
}

if ( $force ) {
    $obj->force("install");
    $obj->uptodate 
        or die "Forced installation of $name appears to have failed";
}
else {
    $obj->install;
    $obj->uptodate 
        or die "Installation of $name appears to have failed";
}
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