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

sub set {
    my( $pos, $newval )= @_;
...
print $x, $/;                       # prints "two"
print set( 2, "hello", "goodbye" ); # dies with:
# Modification of a read-only value attempted

($a,undef,$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