How do I truncate a UNIX binary log file using command line options?

You can use perl as follows to truncates the file opened on FILEHANDLE, or named by EXPR, to the specified length. You can also use truncate command as discussed below.

perl -e'truncate "filename", LENGTH'
perl -e'truncate "foo", 122455'

truncate UNIX command

Various version of UNIX and Linux like operating system comes with the truncate command to shrink or extend the size of a file to the specified size:

truncate -s SIZE fileName
truncate -s 2000KB fileName
truncate -s 2000MB fileName
truncate -s 20GB fileName

The SIZE may also be prefixed by one of the following to adjust the size of each FILE based on their current size:

          +  => extend by
          -  => reduce by
          <  => at most
          >  => at least
          /  => round down to multiple of
          %  => round up to multiple of

The above syntax is based on GNU truncate command. See local man page for more info:
$ man truncate