use Scalar::Util 'readonly';

sub is_const { readonly($_[0]); }

print "undef is readonly\n" if readonly(undef);
print "undef not detected in is_const\n" unless is_const(undef);
for $x (4, "a", undef)
  { print "$x ";$x="VAR" unless readonly($x); print $x,"\n"; }
for $x (4, "a", undef)
  { print "$x ";$x="VAR" unless is_const($x); print $x,"\n"; }