为什么这两个变量 `${ "_<$filename" }` 和 `${ "::_<$filename" }` 不相等？

这个问题与this one.

你说这两个是一样的：

${ "::_<$filename" }                       # Not allowed under strict.
${ "_<$filename" }     # In main package   # Not allowed under strict.

但接下来怎么可能呢？

if( ${ "_<$filename" } ne ${ "::_<$filename" } ) {
    print "MISMATCH\n";
    print ">>${ '_<$filename' }<<\n>>${ '::_<$filename' }<<\n";
}

output:

MISMATCH
>><<
>><<

这发生在$filename is /home/kes/work/projects/tucha/monkeyman/app/maitre_d/../..//local/lib/perl5/Specio/Constraint/Role/Interface.pm

UPD
with "引用为@HåkonHægland建议

    print "MISMATCH\n";
    print ">>${ \"_<$filename\" }<<\n>>${ \"::_<$filename\" }<<\n";
    Devel::Peek::Dump( ${         "_<$filename"  } );
    Devel::Peek::Dump( ${       "::_<$filename"  } );
    Devel::Peek::Dump( ${     $::{"_<$filename"} } );
    Devel::Peek::Dump( ${ ${'::'}{"_<$filename"} } );

输出是：

MISMATCH
>><<
>>/home/kes/work/projects/tucha/monkeyman/app/maitre_d/../..//local/lib/perl5/Specio/Constraint/Role/Interface.pm<<
SV = PV(0xfb84520) at 0xff527a0
  REFCNT = 1
  FLAGS = ()
  PV = 0
SV = PV(0x2e43d80) at 0x2f40400
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x2edc4a0 "/home/kes/work/projects/tucha/monkeyman/app/maitre_d/../..//local/lib/perl5/Specio/Constraint/Role/Interface.pm"\0
  CUR = 111
  LEN = 113
  COW_REFCNT = 0
SV = PV(0x2e43d80) at 0x2f40400
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x2edc4a0 "/home/kes/work/projects/tucha/monkeyman/app/maitre_d/../..//local/lib/perl5/Specio/Constraint/Role/Interface.pm"\0
  CUR = 111
  LEN = 113
  COW_REFCNT = 0
SV = PV(0x2e43d80) at 0x2f40400
  REFCNT = 1
  FLAGS = (POK,IsCOW,pPOK)
  PV = 0x2edc4a0 "/home/kes/work/projects/tucha/monkeyman/app/maitre_d/../..//local/lib/perl5/Specio/Constraint/Role/Interface.pm"\0
  CUR = 111
  LEN = 113
  COW_REFCNT = 0

从输出中我们可以看到主题变量是不同的：

SV = PV(0xfb84520) at 0xff527a0
SV = PV(0x2e43d80) at 0x2f40400

谢谢@HåkonHægland。我错过了:: mean main包裹。

我被以下事情搞乱了：

以下是等效的：

${ "::_<$filename" }                       # Not allowed under strict.
${ "_<$filename" }     # In main package   # Not allowed under strict.

现在我明白这意味着

${ "::_<$filename" }   # This refers to a var in main package
${ "_<$filename" }     # This refers to a var in CURRENT! package