Well, I looked inside Tk::Adjuster,

you can tweak position with  $adjuster->delta_width(-120); but its kinda clumsy,

so you can also do what delta_width ultimately does which is slightly less clumsy  $adjuster->slave->GeometryRequest( 120, $adjuster->slave->height);

update:  $adjuster->slave->GeometryRequest( $adjuster->parent->width/6 , $adjuster->slave->height);