my $filepath = "/tmp/fifo";
logInFIFO('log line example');

sub logInFIFO 
{
  my $logline = shift;
  my $fifolines = 5;
  my @log;

  if (-e $filepath) {
    open(IN, "< $filepath") || die $!;
    @log = (<IN>);
    close IN;
  }

  # transform and validate $logline here

  push @log, "$logline\n"; 
  shift @log while ($#log > $fifolines-1);
  eval
  {
    open(OUT, ">$filepath") || die $!;
    print OUT @log;
    close(OUT) || die $!;
  };

  return $@ ? warn($@) && 0 : 1;
}