In Operators§

See primary documentation in context for infix andthen.

The andthen operator returns Empty if the first argument is undefined, otherwise the last argument. The last argument is returned as-is, without being checked for definedness at all. Short-circuits. The result of the left side is bound to $_ for the right side, or passed as arguments if the right side is a Callable, whose count must be 0 or 1.

A handy use of this operator is to alias a routine's return value to $_ and to do additional manipulation with it, such as printing or returning it to caller. Since the andthen operator short-circuits, statements on the right-hand side won't get executed, unless left-hand side is defined (tip: Failures are never defined, so you can handle them with this operator).

"does this match?" ~~ /this/ andthen {
    .put;
    my $thing-to-do-if-match-succeeds;
    ...
} # OUTPUT: «this␤»

"does that match?" ~~ /this/ andthen put 'oops';
# NO OUTPUT

The andthen operator is a close relative of the with statement modifier, and some compilers compile with to andthen, meaning these two lines have equivalent behavior:

.say with 42;
42 andthen .say;