How can I split a Perl string only on the last occurrence of the separator?

my $str="1:2:3:4:5"; my ($a,$b)=split(':',$str,2);

In the above code I have used limit as 2 ,so $a will contain 1 and remaining elements will be in $b. Like this I want the last element should be in one variable and the elements prior to the last element should be in another variable.

Example

$str = "1:2:3:4:5" ; 
# $a should have "1:2:3:4"  and $b should have "5" 
$str =  "2:3:4:5:3:2:5:5:3:2" 
# $a should have "2:3:4:5:3:2:5:5:3" and $b should have "2"