Make Vim ignore first character in line while indenting

Question

Whenever I have to edit Perl Mason files, I always have problems indenting lines due to Perl code starting with %. For example:

<div>
    <div>
%       if( !$something ) {
            <strong><% $title %></strong>
%       }
    </div>

Any idea how I can tell Vim to ignore the % at the beginning of the line and indent like it wasn't there?

I'm using https://github.com/aming/vim-mason to support the mixed Perl/HTLM syntax, but I don't think it changes anything.