my @y= (undef)= (3,4);

##</code><code>##

sub set {
    my( $pos, $newval )= @_;
    $_[$pos]= $newval;
    return $_[$pos];
}
print set( 2, "hello", undef ), $/; # prints "hello"
my $x= "one";
print set( 2, "two", $x ), $/;      # prints "two", changes $x
print $x, $/;                       # prints "two"
print set( 2, "four", $x.3 ), $/;   # prints "for", $x unchanged
print $x, $/;                       # prints "two"
print set( 2, "hello", "goodbye" ); # dies with:
# Modification of a read-only value attempted

##</code><code>##

($a,undef,$b)= @x;    # [undef] is copied
($a,"hi",$b)= @x;     # "hi" is aliased and so this dies
@x= ($a,undef,$b);    # [undef] is copied so $x[1] is modifiable
@x= ($a,"hi",$b);     # "hi" is copied so $x[1] is modifiable