Here is my solution:

#!/usr/bin/perl -w

use strict;

my $searchstring = "aaaa OR bbbb OR cccc OR dddd";

$searchstring =~ s/(\w+) ?(OR|NOT|AND)|(\w+)$/\(fieldname \= '$1$3'\) 
+$2/g;

print "SELECT * FROM tablename WHERE $searchstring\n";

__OUTPUT__
Use of uninitialized value in concatenation (.) or string at a.pl line
+ 7.
Use of uninitialized value in concatenation (.) or string at a.pl line
+ 7.
Use of uninitialized value in concatenation (.) or string at a.pl line
+ 7.
Use of uninitialized value in concatenation (.) or string at a.pl line
+ 7.
Use of uninitialized value in concatenation (.) or string at a.pl line
+ 7.
SELECT * FROM tablename WHERE (fieldname = 'aaaa') OR (fieldname = 'bb
+bb') OR (fieldname = 'cccc') OR (fieldname = 'dddd')
[download]

But it is not perfect as you can see.