sub log_it
{
  state $fh = do { open my $x, '>>', 'mylog.txt'; $x };
  my $message = shift;
  my $timestamp = time;
  print $fh $timestamp . ": " . $message . "\n";
}
[download]

The above opens the file handle inside the sub. But because it uses a state variable, it keeps the filehandle open between calls.

One object oriented way would be something like this:

{
  package Logger;
  use Any::Mo;
  has handle => (is => 'ro');
  sub log_it {
    my ($self, $message) = @_;
    my $timestamp = time;
    print {$self->handle}
        $timestamp . ": " . $message . "\n";
  }
}

my $logger = Logger->new(handle => $fh);
$logger->log_it("Hello");
$logger->log_it("World");
[download]

A little overload magic can make things more concise...

{
  package Logger;
  use Any::Mo;
  use overload '&{}' => sub {
    my $self = shift;
    return sub { $self->log_it(@_) };
  };
  has handle => (is => 'ro');
  sub log_it {
    my ($self, $message) = @_;
    my $timestamp = time;
    print {$self->handle}
        $timestamp . ": " . $message . "\n";
  }
}

my $logger = Logger->new(handle => $fh);
$logger->("Hello");
$logger->("World");
[download]