$var1 = "2000";
$var2 = "02";
$var3 = "2000";
$var4 = "2";

$var2 does not get interpolated - ie, "02" is not the same as "2" in the context you used. To force numerical context:

$array{$var2} = $array{"02"} = undefined 
$array{$var2+0} = $array{"2"} = 125