Programming Perl, 4th Edition

truncate

truncate FILEHANDLE, LENGTH
truncate EXPR, LENGTH

This function truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length in bytes, not characters. The function raises an exception if ftruncate(2) or an equivalent isn’t implemented on your system. (You can always truncate a file by copying the front of it, if you have the disk space.) The function returns true on success, and undef otherwise.

The behavior is undefined if LENGTH is greater than the current file length. However, on traditional Unix filesystems, it sets the length of the file past the old end, and the kernel returns an intervening, never-written-to data as all zero bytes.

The position in the file of FILEHANDLE is left unchanged. You may wish to call seek before writing to the file after calling truncate on it.