DB<174> use Scalar::Util qw(readonly)
  
  DB<175> sub tst { print readonly($_[0]) ? "r" : "rw" }
 
  DB<176> tst 0
r
  DB<177> tst undef
rw
   DB<179> print readonly(undef) ? "r" : "rw"
r

##</code><code>##

  DB<181> sub tst { $_[0] = 42 }

  DB<182> tst 5
Modification of a read-only value attempted at (eval 119)[/usr/share/perl/5.14/perl5db.pl:640] line 2.

  DB<183> tst undef
 => 42                   # really?