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

sysopen FH, "file.name", O_RDWR | O_CREAT or die "horribly - $!"; # no O_TRUNC!!
flock FH, LOCK_EX or die "screaming - $!";

my @slurp = <FH>;
do_something_with(\@slurp);

seek FH, 0, SEEK_SET;
print FH @slurp;

truncate FH, tell FH;
close FH;