Right, I didn't test very much.   :-) This works correctly and appears to be about 10% faster:

sub useSubstr { 
    my $number = shift;
    my $len = length $number;
    my $offset = 3;
    while ( $offset < $len ) {
        substr $number, -$offset , 0, q{,};
        $offset += 4;
        $len = length $number;
        }
    return $number;
    }
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