open HANDLE, "<", $file or die "Can't open $file for reading: $!";
flock HANDLE, LOCK_SH or die "Can't get a shared lock to $file: $!";

# Open the file without clobbering it.
open WRITER, "+<", $file or die "Can't open $file for writing: $!";
flock WRITER, LOCK_EX or die "Can't get an exclusive lock to $file: $!";
truncate WRITER, 0 or die "Can't truncate $file: $!";