$ perl -wl
$x = "abc";
print "4-arg: ", substr($x="abc", 1, 1, "");
print "3-arg: ", substr($x="abc", 1, 1) = "";
__END__
4-arg: b
3-arg: c

##</code><code>##

sub FETCH { my $self=shift; substr($self->{string}, $self->{length}, $self->{offset}) }
sub STORE { my ($self, $new) = @_; substr($self->{string}, $self->{length}, $self->{offset}, $new) }