I'm not familiar with using regexp's in SQL, but could you not join all the keywords into one regex? Ie, instead of doing

foreach $keyword ( @keywords )
{
  $sql_stmt .= " AND ( PlantDescription REGEXP "s/\$keyword/" );"
}
[download]

you could do

$keywords = join "|", @keywords;
$sql_stmt .= "AND ( PlantDescription REGEXP \"$keywords\" )";
[download]