sub g {
    my ( $a ) = @_;
    sub f { $a };
    return sub { $a = $_[0] };
}

push my @setters, g("Hi\n"), g("Bye\n");

print f; # => Hi
$setters[0]("Bye\n");
print f; # => Bye
$setters[1]("Hi\n");
print f; # => Bye (still)