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my ($last_five) = map scalar reverse, 
                  map /^(?>0*)(\d{1,5})/,
                  map scalar reverse, 
                  $running_total;

my ($last_five) = $running_total =~ /(\d{0,4}[^0\D])0*$/;

my ($last_five) = $running_total =~ /(\d{1,5}(?<!0))0*$/;