gistfile1.pl

use strict;
use warnings;
use Fcntl qw/:DEFAULT :flock/;
use File::stat;

sub _parse_stats_file {
  my ($self) = @_;
  my $stats_fh = $self->_stats_fh();
  my $results = +{};
  while (<$stats_fh>) {
...
  }
  close $stats_fh;
  return $results;
}

sub _stats_file {
  my ($self) = @_;
  return "/tmp/stats"
}

sub _stats_fh {
  my ($self) = @_;
  sysopen my $fh, $self->_stats_file(), O_RDWR|O_CREAT or die $!;
  flock $fh, LOCK_SH or die $!;

  my $size  = stat($self->_stats_file())->size;
  my $mtime = stat($self->_stats_file())->mtime;
  if ($size == 0 or $mtime < time - $self->{expire_sec}) {
    flock $fh, LOCK_EX or die $!;
    $self->_write_stats_file($fh);
  }

  return $fh;
}

sub _write_stats_file {
  my ($self, $fh) = @_;
  seek($fh, 0, 0);
...
  truncate($fh, tell($fh));
  seek($fh, 0, 0);
}