Yes, by placing it first.

my ($last_five) = map scalar reverse, 
                  map /^(?>0*)(\d{1,5})/,
                  map scalar reverse, 
                  $running_total;
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But note that I had to add (?>...) in order to prevent backtracking in the situation where the input is "0000". It's simpler to make sure the last of 5 isn't a 0.

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

my ($last_five) = $running_total =~ /(\d{1,5}(?<!0))0*$/;
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In all of the above solutions, $last_five will be undef if there's no match.

Tested.