# x not defined
print "X not defiined:\t ",scalar @x,"\n" if (!defined(@x));
# given an empty list
@x = x();
print "X empty list:\t ",scalar @x,"\n";
@x = (1,2,3);
# x given a populated list
@x = x();
print "X populated:\t ",scalar @x,"\n";

sub x () {
  return @x if defined(@x) and warn "\@x was already defined";
  return @x if (scalar @x==0) and warn "\@x is 0";
  return wantarray ? () : undef;
}


Output:
X not defiined:  0
@x is 0 at wa.pl line 14.
X empty list:    0
@x was already defined at wa.pl line 13.
X populated:     3