The last command is like the break statement in C (as used in loops); it immediately exits the loop in question. If the LABEL is omitted, the command refers to the innermost enclosing loop. The continue block, if any, is not executed:
LINE: while (<STDIN>) {
last LINE if /^$/; # exit when done with header
#...
}last cannot be used to exit a block that returns a value such as eval {}, sub {} or do {}, and should not be used to exit a grep() or map() operation.
Note that a block by itself is semantically identical to a loop that executes once. Thus last can be used to effect an early exit out of such a block.
See also "continue" for an illustration of how last, next, and redo work.