How about using the powers of join, map and split?

#!/usr/bin/perl -wT
use strict;

my $input = "word+word+word";
my $i=1;
my $string = join '&',
             map {"keyword".$i++."=$_"}
             split /\+/, $input;
print "$string\n";

__END__
keyword1=word&keyword2=word&keyword3=word
[download]

Reading that complicated line from bottom to top... First we split the string into pieces, then we map those pieces to the new format, finally we join them together with a new delimiter.

Update: Just for kicks... this regex seems to work as well:

my $i = '';
(my $string = $input) =~ s/(^|\+)/($i++&&'&')."keyword$i="/ge;
[download]

-Blake