$a = 'string';
if (defined ($a <=> $a)) { print "Oops"; }

##</code><code>##

package Foo;

use overload
  '0+'  => \&as_num;

sub new {
  my $class = shift;
  my $self  = { value => shift };
  bless $self, $class;
}

sub as_num {
  my $self = shift;
  return $self->{value};
}

package main;

my $x = new Foo(3);
my $z = (0+$x); # $x+0 also produces error


##</code><code>##

use Scalar::Util 'looks_like_number';

my $a = [ ];
my $h = { };

print looks_like_number($a), "\n";
print looks_like_number($h), "\n";

##</code><code>##

sub _numberlike {
  return 1, unless defined $_[0];

  # L<perlfunc> manpage notes that NaN != NaN, so we can verify that
  # numeric conversion function works properly along with the
  # comparison operator.
  no warnings;

  return 1 if ((!ref $_[0]) || blessed($_[0])) &&
    eval {
      ((0+$_[0]) == (0+$_[0])) && (($_[0] <=> $_[0])==0)
    };
  return;
}

##</code><code>##

sub _stringlike {
  return 1 unless defined $_[0] && ref $_[0];
  return 1 if (blessed $_[0]) &&
    eval {
      (("$_[0]" eq "$_[0]") && (($_[0] cmp $_[0])==0))
    };
  return;
}