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use warnings;
use strict;
...

    return "Str " . ('x' x (substr ($key, 1) % 100));
}

        Rate  YAML   XML  fast slurp
YAML  21.0/s    --   -8%  -53%  -98%
XML   22.8/s    8%    --  -50%  -98%
fast  45.2/s  115%   98%    --  -96%
slurp 1211/s 5665% 5216% 2583%    --