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这个问题与这个问题有关：Hash keys encoding: Why I get here with Devel::Peek::Dump two different results?
当我取消注释该# utf8::upgrade( $name );行或注释掉该$hash{'müller'} = 'magenta';行时，它会起作用。

#!/usr/bin/env perl
use warnings;
use 5.014;
use utf8;
binmode STDOUT, ':encoding(utf-8)';
use XML::LibXML;

# Hash read in from a file:
# ... 
my %hash = ( 'müller' => 'green', 'schneider' => 'blue', 'bäcker' => 'red' );
# ...

# change or add something
$hash{'müller'} = 'magenta';

# writing Hash to xml file
my $doc = XML::LibXML::Document->new('1.0', 'UTF-8' …