### Makefile.PL

WriteMakefile(
    ...
);

package MY;

# override / modify test target
sub test {
    my $t = shift->SUPER::test(@_);
    $t =~ s|^test ::\K| mymock/mymock |m;  # inserts your prereq
    return $t;
}

# add your own target
sub postamble {
    return <<'MAKE_MOCK';
mymock/mymock : mymock/Makefile
	cd mymock && $(MAKE) all

MAKE_MOCK
}