open OLDDB, '<', $dbname or die ...;
open NEWDB, '>', $tempfile or die ...;
open ARCHIVE, '>>', $archive or die ...;

print ARCHIVE while defined( $_ = <OLDDB> ) 
                and substr( $_, -17, 10 ) lt $targetDate;

print NEWDB; ## Output first 'failing' record to newdb
print NEWDB while <OLDDB>;

close for OLDDB, NEWDB, ARCHIVE;
unlink $dbname;
rename $tempfile, $dbname;