open my $FH, ">>foo" or die "can't open";
flock $FH, LOCK_EX or die "no lock";
seek ($FH, 0, 2);

##</code><code>##

open my $LOCK, "<foo" or die "can't open for locking";
flock $LOCK, LOCK_EX or die "Can't lock";
open my $FH, ">foo" or die "can't open for writing";
print $FH "I own this file\n";
close $FH;
close $LOCK;