#!/usr/bin/perl

use strict;
use warnings;

while (<DATA>) {
    s/(<%(?:[^%]+|%[^>])*%>)|'/$1 || ""/eg;
    print;
}


__DATA__
Leave this line alone.
Remove this quote ' and these two quotes '' from this line.
Leave <% the first quote ' %> alone, but remove the second ' one.
Remove from <% an open but not closed ' tag.
' Leading quote?
<% Multiple ' tags %> ' on one ' <% line ''' work  %> too ' !