perl -w -le 'sub is_const { !eval { ($_[0]) = $_[0]; 1; } }
 print "undef is ",(is_const(undef)?"":"not")," a constant"'
undef is not a constant

perl -MDevel::Peek -le 'Dump(undef)'
SV = NULL(0x0) at 0x8120d38
  REFCNT = 2147483622
  FLAGS = (READONLY)