s{(?mx:
    # Explicitly set /mx for this scope. This puts the modifiers right up where they are being used instead of at the end of the regex.
    ^\[$
       ((?s:.*?)) # Non-greedy match of multiple lines
    ^\]$
  )}{
    my $tmp = $1;
    $tmp =~ s{^([^\n]+)$}{<li>$1</li>}mg; # Added /m and fixed closing bracket. Preferred [^\n] to . for explictness.
    "<ul>$tmp</ul>"
}eg;