my $open = qr{
  < [a-zA-Z][a-zA-Z0-9]*
  (?:
    \s+ \w+
    (?: \s* = \s*
      (?: "[^"]*" | '[^']*' | [^\s>]* )
    )?
  )*
  \s*
  >
}x;

##</code><code>##

my $close = qr{
  < / \s* [a-zA-Z][a-zA-Z0-9]* \s* >
}x;  

##</code><code>##

# the following are comments:
# <!-- ab -- cd -->   <!-- ab -->    <!---->
# <!-- ab -- cd -- >  <!-- ab -- >   <!---- >

my $comment = qr{
  <!--              # <!--
  [^-]*             # 0 or more non -'s
  (?:               
    (?! -- \s* > )    # that's not --, space, then >
    -                 # a -
    [^-]*             # 0 or more non -'s
  )*                # 0 or more times
  -- \s* >          # --, space, then >
}x;

##</code><code>##

while ($HTML =~ /\G($open|$close|$comment|[^<]+)/g) {
  # do something with $1
}

##</code><code>##

# requires the (?{...}) structure
use re 'eval';

while ($HTML =~ m{
  \G
  (
    $open    (?{ $STATE = 'open'    }) |
    $close   (?{ $STATE = 'close'   }) |
    $comment (?{ $STATE = 'comment' }) |
    [^<]+    (?{ $STATE = 'TEXT'    })
  )
}xg) {
  # do something with $1 and $STATE
}