Mapping key `=` must be dumped with quotes

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Otherwise, when dumped as a plain scalar, it's the special "default" key:

use YAML;
use Data::Dumper;
my $hashref = {"=" => "bar"};
my $yaml = Dump $hashref;
say $yaml;
my $load = Load $yaml;
say Dumper $load;
__END__
---
=: bar

$VAR1 = {
          'YAMLVALUE' => 'bar'
        };

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Released YAML 1.24_002