use Fcntl qw(:DEFAULT :flock :seek);

sysopen FILE, $mailclicks, O_RDWR | O_CREAT or die "couldn't open $mailclicks: $!";
flock FILE, LOCK_EX or die "failed acquiring lock on $mailclicks: $!";

my @slurp = <FILE>;

# do something with @slurp

seek FILE, 0, SEEK_SET; # go back to top of file
print FILE @slurp;

truncate FILE, tell FILE; # because the file may be smaller than it was before
close FILE;