`perldoc -q comma' yields

Found in C:\Perl\lib\pod\perlfaq5.pod
  How can I output my numbers with commas added?

            This one will do it for you:

                sub commify {
                    local $_  = shift;
                    1 while s/^([-+]?\d+)(\d{3})/$1,$2/;
                    return $_;
                }

                $n = 23659019423.2331;
                print "GOT: ", commify($n), "\n";

                GOT: 23,659,019,423.2331

            You can't just:

                s/^([-+]?\d+)(\d{3})/$1,$2/g;

            because you have to put the comma in and then recalculate 
+your
            position.

            Alternatively, this code commifies all numbers in a line
            regardless of whether they have decimal portions, are prec
+eded
            by + or -, or whatever:

                # from Andrew Johnson <ajohnson@gpu.srv.ualberta.ca>
                sub commify {
                   my $input = shift;
                    $input = reverse $input;
                    $input =~ s<(\d\d\d)(?=\d)(?!\d*\.)><$1,>g;
                    return scalar reverse $input;
                }
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