use strict;
use Fcntl qw(:flock);

my $file = 'data.file';
my $SEMAPHORE = "$file.lck";

open(S, ">$SEMAPHORE) || die "Foo ($!)";
flock(S, LOCK_EX);
open(FH, ">$file") || die "Foofoo ($!)";
# Muck with file
close FH;
close S;