Here is a solution. The basic idea to count the times we of the found pattern as we scan the string from left to right. Although we can compact the code into 2 short lines, it is shown bellow longer and verbose.

# $n is the nth wanted occurence, as indicated by the question.
# $i holds how many times we found the pattern

my $str     = ',n ,pple , d,y';
my ($n, $i) = 4;


$str =~ / ,                  # we are searching for the , pattern 
          (?{ $i++})         # we found it again , increament $i 
        /xgc                 # retain last occurance on failure
        for (1..$n);

# print position in string
print +($n == $i)   ?  pos($str) - 1
                    : 'not found';
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