It's perfectly valid result.

m/^(?:(\d+)|::)*\1$/x

#which means the same as:

m/^            

     (\d+)*   #capture digits from the beginning of string
     \1       #than match the same digit as $1 in the end
              #and the backtracking (in reverse order) ends happily
              #where $1 is the digit before the last one
$/;
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So for example if you have string like '12312377' then $1 will be 7.