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

...
ok( ! is_code( $undef   ), 'undef is not code' );
ok( ! is_code( 'string' ), 'string is not code' );
ok( is_code( 'real_sub' ), 'string is code' );

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

use strict;
use warnings;
...
    ok( ! $side_effect,            "$name: no side effect" );
    is( $undef, undef,             "$name: \$undef is still undef" );
}