$ perl -wlne'eval;print$@if$@ # interactive perl'
use Scalar::Util 'dualvar';
$x = dualvar(256, sprintf "%x", 256);
print $x;
100
# correct hexadecimal string
print 0+$x;
256
# correct numeric value

##</code><code>##

$x .= "foo";
print 0+$x;
Argument "100foo" isn't numeric in addition (+) at (eval 9) line 1, <> line 8.
100
print $x;
100foo
$x = dualvar(256, '100');
$x += 3;
print 0+$x;
259
print $x
259