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

You could use pattern matching instead of split():

my ($a, $b) = $str =~ /(.*):(.*)/;

The first group captures everything up to the last occurence of ':' greedily, and the second group captures the rest.

In case the ':' is not present in the string, Perl is clever enough to detect that and fail the match without any backtracking.

split(/:([^:]+)$/, $str)