Append extra character c at the end of string s to make sure its length is at least m. The length is the number of characters, not the number of bytes.
String $c (one character) is repeated $m - length $s times. Perl generates an empty string if the repeat count is zero or negative. The result is appended to $s (by the .= operator, similar to +=).
The . operator concatenates; the x operator repeats a string on the left by the count on the right. For compactness, ? : is used instead of if-else. A more idiomatic approach would avoid calling length() twice.
