** Using WeBWorK **

Display, centered or not, of the following aligned list

$al = new_aligned_list(ans_rule_len=>15, spacing=>'5', tex_spacing=>'5 pt');
$al -> qa(
    'amplitude $SPACE' , $AMP -> cmp() ,
    'period' , $PER -> cmp() ,
);

shows same-width spacing around = for the first line but not the second:
    amplitude  =  [box]
            period=  [box]

Although coding is cleaner for the second line, I have been using the first line's hack because no value for the spacing parameter has given the prettier display.  Are there alternative ways to get same-width spacing around =?