join
See the current Perl documentation for join.
Here is our local, out-dated (pre-5.6) version:
join - join a list into a string using a separator
join EXPR,LIST
Joins the separate strings of LIST into a single string with fields separated by the value of EXPR, and returns the string. Example:
$_ = join(':', $login,$passwd,$uid,$gid,$gcos,$home,$shell);
See split.