'I' => {
     $a => [4,10,200],
     $b => [75,350],
. . .

##</code><code>##

'I' => {
    4 => $a,
    10 => $a,
    200 => $a,
    75 => $b,
    350 => $b,
. . .

##</code><code>##

'I' => {
     $a => [10,200],
     $b => [75,200,350],
. . .

##</code><code>##

$interval = 500;
'I' => {
     $a => [200,500,800],
. . .