When used in programming, this usually means to tack one thing onto the end of another thing.
Users of c can use strcat(string1, string2)
where string1 and string2 are pointers to the beginning of arrays of type char. This cuts the terminating null character off the first string, then copies in the second string (including the null char). If the first string isn't big enough, the function pukes. Otherwise, it returns the pointer to the first string. Oh, and include string.h to use this function.
As with most things, perl is a lot easier. Just do something like $string3 = $string1 . $string2