my @parsed;

while ($asp =~ /\G ((?: [^<]+ | <(?!%) )*) (?: <%(.*?)%> | ((?=<%)) )? /gsx) {
    $1 and push @parsed, [ html => $1 ];
    $2 and push @parsed, [ asp  => $2 ];
    defined $3 and die "Unclosed ASP code block near '",
        $asp =~ /\G(<%\s*\n?.*)/g, "'.\n";
}

##</code><code>##

<% foo = "a mere %> breaks either simple minded solution." %>