@x=(\$_,\$_);
my $initial_value = $x[0];
$_=10;
print "\$x[$_] = ${$x[$_]}\n" for 0..$#x;
die "But \$x[0] did not change.\n" if $x[0] eq $initial_value;
$_=20;
print "\$x[$_] = ${$x[$_]}\n" for 0..$#x;
die "But \$x[0] did not change.\n" if $x[0] eq $initial_value;

##</code><code>##

print "\$x[$_] = ${$x[$_]}\n" for 0..$#x;
                 ^^      ^
LHS is $foo, RHS is $$foo. The output is very misleading.

print "\${\$x[$_]} = ${$x[$_]}\n" for 0..$#x;

LHS is $$foo and RHS is $$foo. But we're interested in $foo.

print "\$x[$_] = $x[$_]\n" for 0..$#x;

LHS is $foo and RHS is $foo. Output is correct and shows that the needed change did not happen.