...if regex...
   seek(LOG,0,0) || die "Seek error: $!";
   ...process line...
   truncate(LOG,tell(LOG))

##</code><code>##

   ...if regex...
   $pos = tell(LOG)
   seek(LOG,0,0) || die "Seek error: $!";
   ...process line...
   truncate(LOG,$pos)

##</code><code>##

truncate(LOG,$pos-$stringlength);