$input = "word+word+word";

##</code><code>##

keyword1=word&keyword2=word&keyword3=word

##</code><code>##

$string = "";
$wordnum = 0;
$input = "word+word+word";
(@keywords) = split(/\+/, $input);

foreach $keyword (@keywords){
$wordnum++;
$string .= qq~keyword_$wordnum=$keyword~;
}