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

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

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