Documentation for syntax return-rw assembled from the following types:

language documentation Control flow

From Control flow

(Control flow) control flow return-rw return-rw

The sub return will return values, not containers. Those are immutable and will lead to runtime errors when attempted to be mutated.

sub s(){ my $a = 41; return $a };
say ++s();
CATCH { default { say .^name, ': ', .Str } };
# OUTPUT: «X::Multi::NoMatch.new(dispatcher …

To return a mutable container, use return-rw.

sub s(){ my $a = 41; return-rw $a };
say ++s();
# OUTPUT: «42␤»

The same rules as for return regarding phasers and control exceptions apply.