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

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

truncate(LOG,$pos-$stringlength);