Hi Colin,

You probably want to use sysopen rather than open. The canonical way to do this (on a single output file) is something along the following lines:

use Fcntl qw(:DEFAULT :flock);

sysopen(OUT, $outfile, O_WRONLY | O_CREAT)
   or die "Cannot open $outfile for writing:$!\n";
flock(OUT, LOCK_EX)
   or die "Cannot get a lock on $outfile:$!\n";
truncate(OUT, 0)
   or die "Cannot truncate $outfile:$!\n";
[download]

There are a couple of very good discussions regarding file locking in the Camel (3rd Ed.). If you happen to have a copy, look in Chapters 16.2.1 and 23.2.2.

Otherwise, you can refer to the docs for sysopen and flock.
perldoc -q lock also gives some useful examples.

Hope this helps,
Darren :)