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_classes";
}

##</code><code>##

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

##</code><code>##

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

##</code><code>##

BEGIN {
    # YAML classes to attempt to load (in order of preference)
    my @yaml_classes = qw{YAML::XS YAML::Syck YAML::Tiny YAML};
    # Expecting to import these functons from whatever YAML class is loaded
    my @yaml_imports = qw{Dump Load};

    my $yaml_class = first { eval "use $_ (\@yaml_imports); 1" } @yaml_classes
        or die "Unable to load any of these YAML classes: @yaml_classes",
                " (using import list: @yaml_imports)";
}

##</code><code>##

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

    # Only expecting whatever YAML class is loaded to export these functions
    my @yaml_funcs = qw{Dump Load};
    no strict 'refs';
    my %yaml_exports = map { $_ => 1 } @{$yaml_class . "::EXPORT"};
    $yaml_exports{$_} or die "$yaml_class doesn't export '$_'" for @yaml_funcs;
}