$old_db     = "emdb1";
$new_db     = "kuku"; 
$string     = "AUTHORIZATION,libctl62-m.sl,CONTROLM,emdb1,b02ed6600206ca161d165355c67b72073b6123d6838f1f68203b78cb9c4c63c9,emdb2,,;" ; 
$str_regexp = '"$1$2$3$new_db,"' ;

$string     =~ s/([^,]*,)([^,]*,)([^,]*,)([^,]*,)/$str_regexp/ee; 

print "$string\n";