truncate
See the current Perl documentation for truncate.
Here is our local, out-dated (pre-5.6) version:
truncate - shorten a file
truncate FILEHANDLE,LENGTH
truncate EXPR,LENGTH
Truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. Produces a fatal error if truncate isn't implemented on your system. Returns TRUE if successful, the undefined value otherwise.