use Fctl qw(:flock);

my $fh = new IO::File("file1","r");
flock($fh,LOCK_EX) or die "can't get lock";
my $fh2 = new IO::File("file2","w");
# read fh and write fh2 (ie do a filter)
# move fh2 over fh
# rename("file2","file1"); doesn't work
# File::Copy's move($fh2,$fh1); doens't work
flock($fh,LOCK_UN);