I'd go with something similar to the above, but remove some stuff:

my $message = '';

if ( $char_keywords > 950 ) {
  $message = "You had $char_keywords chars. Max is 950";
}

if ( failrepeattest() ) {
  $message .= '<br />' if $message;
  $message .= 'Repeated keyword error message.';
}

if ( $message ) {
  print qq(<td><font color="red">Keywords: </font></td>);
  print qq(<td><font color="red">$message</font><br>);
} else {
  print qq(<td>Keywords: </td>);
}
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You don't need the $error flag. The $message will be empty (false) if there are no errors. I've also taken out the length() check on $message for the same reason .. if the message has length then testing it in this context will return true and the <br> tag will be output.