Icky hack (local ... for ...;):

sub Dump {
    my @opts;
    for my $option (@dump_options) {
        no strict 'refs';
        my $ref = \${$implementation.'::'.$option};
        push @opts, [ $ref =>
            defined($$ref) ? $$ref : ${"YAML::$option"},
        ];
    }
    local ${$_->[0]} = $_->[1] for @opts;
    return &{"$implementation\::Dump"};
}
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Sticking to something that won't break:

{
    package YAML::OnScopeExit;
    sub DESTROY { ${ shift() }->() }
}

sub _on_scope_exit(&) {
    my ($sub) = @_;
    return bless(\$sub, 'YAML::OnScopeExit');
}

sub Dump {
    my @opts;
    for my $option (@dump_options) {
        no strict 'refs';
        my $ref = \${$implementation.'::'.$option};
        push @opts, [ $ref =>
            defined($$ref) ? $$ref : ${"YAML::$option"},
            $$ref,
        ];
    }

    my $sentinel = _on_scope_exit {
        ${$_->[0]} = $_->[2] for @opts;
    };

    ${$_->[0]} = $_->[1] for @opts;

    return &{"$implementation\::Dump"};
}
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Both untested.