Kinda messy. I know there's a better way to do it, but I don't have the reference book with me. Here's what I came up with:

package A;

sub bar { return "BAR"; }

sub AUTOLOAD {
    my (@objpath) = split(/::/, $AUTOLOAD);
    my $fname = pop @objpath;
    return if $fname eq 'DESTROY';
    if ( $fname eq 'baz' ) {
        my $subr = <<EOSUB;
sub $AUTOLOAD {
     return "FOO";
}
EOSUB
    eval $subr;
    goto &$AUTOLOAD;
    return $subr;
    }
    return;
}

package B;

@ISA = qw( A );

sub new { return bless {}, shift; }


sub AUTOLOAD {
    my (@objpath) = split(/::/, $AUTOLOAD);
    my $fname = pop @objpath;
    return if $fname eq 'DESTROY';
    if ( $fname eq 'foo' ) {
        my $subr = <<EOSUB;
sub $AUTOLOAD {
     return "FOO";
}
EOSUB
    eval $subr;
    goto &$AUTOLOAD;
    } else {
        return &{"A::$fname"};
    }
    return;
 }

package main;

my $b = new B;

print "foo: ", $b->foo, "\n";
print "bar: ", $b->bar, "\n";
print "baz: ", $b->baz, "\n";
[download]

The biggest change I made was adding an else clause to B::AUTOLOAD(). If the appropriate subroutine isn't found in package B, it constructs a symbolic reference to a function in A. Since that function may not be there, it's trapped by AUTOLOAD.

I have no idea whether or not this gets caching, but I'll check. It's not very clean or elegant or safe, so I wouldn't use it except in an emergency. Still, it's One Way To Do It.