`more_headers` with `eval` filter will throw warning of 'uninitialized value $key'

this is my t directory layout:

└── t
    └── Var
        ├── Conf.pm
    ├── test.t

the content of Conf.pm

package Var::Conf;

use Exporter qw(import);

our @EXPORT_OK = qw(Headers);


our $Headers = q{
Cookie: sessionId=0ca3eafb0a8ffc8146ac0609897d6a24; controlId=7cf98c6218e2f4154ff74b69a6fa3f5d
};

1;

the content of test.t

use Test::Nginx::Socket::Lua no_plan;
use Cwd qw(cwd abs_path);

use File::Basename qw(dirname);
use lib dirname(abs_path $0);
use Var::Conf qw(Headers);

$ENV{TEST_NGINX_CODE_PWD} = cwd();

#repeat_each(2);

no_long_string();

run_tests();

__DATA__

=== TEST 1: basic resty test
--- http_config
    init_worker_by_lua_block {
        print("init")
    }

--- config
    location = /t {
        echo ok;
    }

--- more_headers eval: $Var::Conf::Headers

--- request
GET /t
--- response_body
ok

then , run the cmd: prove t/test.t
will give the warning of uninitialized value

Use of uninitialized value $key in lc at /usr/local/share/perl/5.18.2/Test/Nginx/Socket.pm line 301, <DATA> line 1.
Use of uninitialized value $key in concatenation (.) or string at /usr/local/share/perl/5.18.2/Test/Nginx/Socket.pm line 306, <DATA> line 1.
t/test.t .. ok   
All tests successful.
Files=1, Tests=2,  0 wallclock secs ( 0.02 usr  0.01 sys +  0.18 cusr  0.04 csys =  0.25 CPU)
Result: PASS

if i replace the --- more_headers eval: $Var::Conf::Headers in test.t, with

--- more_headers
    Cookie: sessionId=0ca3eafb0a8ffc8146ac0609897d6a24; controlId=7cf98c6218e2f4154ff74b69a6fa3f5d

the warning message gone.

============================================
OS: Ubuntu 14.04
Test:Nginx v0.25

perl -v give me this:

This is perl 5, version 18, subversion 2 (v5.18.2) built for x86_64-linux-gnu-thread-multi

nginx -v output:

nginx version: openresty/1.9.7.4

Reproduce the warning with t/test.t and the eval form of the more_headers directive, then inspect Test/Nginx/Socket.pm around lines 301 and 306. Compare this with the literal more_headers case; done means the eval form processes the header without uninitialized-value warnings while the test still passes.