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BEGIN {
    my @yaml_classes = qw{YAML::XS YAML::Syck YAML::Tiny YAML};
    first { eval "use $_; 1" } @yaml_classes
        or die "Unable to load any of these YAML classes: @yaml_classe
+s";
}

Unable to load any of these YAML classes: YAML::XS YAML::Syck YAML::Ti
+ny YAML at ...
BEGIN failed--compilation aborted at ...

my $yaml_class;
BEGIN {
...
        or die "Unable to load any of these YAML classes: @yaml_classe
+s";
}
print $yaml_class;

BEGIN {
    # YAML classes to attempt to load (in order of preference)
...
        or die "Unable to load any of these YAML classes: @yaml_classe
+s",
                " (using import list: @yaml_imports)";
}

BEGIN {
    my @yaml_classes = qw{YAML::XS YAML::Syck YAML::Tiny YAML};
...
    my %yaml_exports = map { $_ => 1 } @{$yaml_class . "::EXPORT"};
    $yaml_exports{$_} or die "$yaml_class doesn't export '$_'" for @ya
+ml_funcs;
}