perl - 为什么 `exists` 修改我的常量？

Question

exists 功能可以unexpectedly autovivify entries在哈希中。

令我惊讶的是，这种行为也适用于常量:

use strict;
use warnings;
use Data::Dump 'dump';

use constant data => {
                       'foo' => {
                                  'bar' => 'baz',
                                },
                       'a'   => {
                                  'b'   => 'c',
                                }
                     };

dump data;   # Pre-modified

print "No data for 'soda->cola->pop'\n" unless exists data->{soda}{cola}{pop};

dump data;   # data->{soda}{cola} now sprung to life

输出

{ a => { b => "c" }, foo => { bar => "baz" } }
No data for 'soda->cola->pop'
{ a => { b => "c" }, foo => { bar => "baz" }, soda => { cola => {} } }

我怀疑这是一个错误。这是特定于 5.10.1 的东西，还是其他版本的 Perl 行为类似？

Answer 1

这是记录在案的行为。 perldoc constant说:

Even though a reference may be declared as a constant, the reference may point to data which may be changed, as this code shows.

use constant ARRAY => [ 1,2,3,4 ];
print ARRAY->[1];
ARRAY->[1] = " be changed";
print ARRAY->[1];

它是常量的引用，而不是它所指的对象。