$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/e; 
print "${string}\n";