s{ <\?nm(             # opening angle bracket
    (?:   # Non-backreffing grouping paren
    [^>'"] *  # 0 or more things that are neither > nor ' nor "
    |     # or else
    ".*?" |     # a section between double quotes (stingy match)
    |     # or else
    '.*?' |     # a section between single quotes (stingy match)
    ) +              # repetire ad libitum
    #  hm.... are null tags <> legal? XXX 
    )\?>          # closing angle bracket
}{my $var = $1; my_handler($r, $var)}geisx;  # mutate