Which library is capable of parsing this?

None

YAML is weak :) Re: Loading multiple files with YAML::XS (named streams)

#!/usr/bin/perl --
use strict;
use warnings;

use Module::Load qw' load ';
use YAML::Any();
my $yaml = q{
# Explicitly typed set.
baseball players: !!set
  ? Mark McGwire
  ? Sammy Sosa
  ? Ken Griffey



# Flow style
baseball teams: !!set { Boston Red Sox, Detroit Tigers, New York Yanke
+es }

};


MOD_YAML :
for my $modYaml ( qw[
    YAML::XS
    YAML::XS::LibYAML
    YAML::Syck
    YAML::Old
    YAML
    YAML::Tiny  ] ){
#~     eval { load $modYaml; 1 } or do { warn $@; next MOD_YAML };
    eval { load $modYaml; 1 } or next MOD_YAML;
    no strict 'refs';
    local *Dump = \&{"$modYaml\::Dump"};
    local *Load = \&{"$modYaml\::Load"};
    local *{"$modYaml\::CompressSeries"} = 0;
    local *{"$modYaml\::Indent"} = 5;
    local *{"$modYaml\::UseFold"} = 5; # no visible effect
    local *{"$modYaml\::UseBlock"} = 1; # 5; # no visible effect, 1 or
+ 5
    print "\n# Using $modYaml\n";
    print eval { Dump( Load( $yaml ) ) } || $@;
    print "\n\n";
}
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