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

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

# the following are comments:
# <!-- ab -- cd -->   <!-- ab -->    <!---->
...
  )*                # 0 or more times
  -- \s* >          # --, space, then >
}x;

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

# requires the (?{...}) structure
use re 'eval';
...
}xg) {
  # do something with $1 and $STATE
}