use Test::More 'tests' => 5;

sub is_code {
    no warnings qw( void uninitialized );
    return eval { defined &{$_[0]} };
}

sub real_sub { die 'real sub called' }
my $sub_ref = sub { die 'sub ref called' };
my $undef;

ok( is_code( \&real_sub ), 'real sub ref is code' );
ok( is_code( $sub_ref   ), 'lexical sub ref is code' );
ok( ! is_code( $undef   ), 'undef is not code' );
ok( ! is_code( 'string' ), 'string is not code' );
ok( is_code( 'real_sub' ), 'string is code' );