# Open the file for locking purposes only
open my $SEM, "the_file";
flock($SEM, LOCK_EX);       # lock it

# Process it
open my $FH, "the_file";
my @lines = <$FH>;
open $FH, ">the_file";
push @lines, "new line\n";
print $FH for (@lines);
close $FH;

close $SEM;                # close and unlock semaphore