## ... circa line 152 of HTML/TreeBuilder.pm
$self->{'_ignore_unknown'}    = sub { 1 };


## ... circa line 660 in HTML/TreeBuilder.pm
if( $self->{ '_ignore_unknown' }->( $tag ) ) {
  print $indent, " * Ignoring unknown tag \U$tag\E\n" if DEBUG
  $self->warning("Skipping unknown tag $tag");
  return
}

## ... later in your code
$tree->ignore_unknown( sub { return 1 if $_[0] !~ /^mig:/ } );