my $html =<<'HTML';
<ul>
<li>question 1</li>
<li>question 2</li>
</ul>

<ul>
<li>answer 1</li>
<li>answer 2</li>
</ul>

<ul>
<li>question 3</li>
<li>question 4</li>
</ul>

<ul>
<li>answer 3</li>
<li>answer 4</li>
</ul>
HTML


my(@questions, @answers);
my @arr = \(@questions, @answers);

while ($html =~ m{<ul> (.*?) </ul>}xmsg ) {

    my $list = $1;

    while ($list =~ m{<li> (.*?) </li>}xmsg ) {
        my $question_or_answer = $1;
        push @{$arr[0]}, $question_or_answer;
    }
    
    (@arr[0], @arr[1]) = (@arr[1], @arr[0]);
}

print "@{$arr[0]} \n";
print "@{$arr[1]} \n";

--output:--
question 1 question 2 question 3 question 4 
answer 1 answer 2 answer 3 answer 4