sub MyRecs::foo {
    my( $ro, $sham, $bo ) = @_;
    die "sham must sham " if not $sham =~ /sham/i;
    ...
}
sub MyRecs::Lil::foo {
    MyRecs::foo( unpack "n3A*", ... );
}
sub MyRecs::Big::foo {
    MyRecs::foo( unpack "v3A*", ... );
}