my $fh = undef;
print "1.fh is now $fh\n";
printout("some lines", \$fh);     
print "2.fh is now $fh\n";
printout("some lines", \$fh);
print "3.fh is now $fh\n";
$fh = undef; # <<<< very important, else redirects STDOUT to xxx
open $fh, '>', 'xxx';
printout("some lines", \$fh);
print "4.fh is now $fh\n";
close $fh;

sub printout {
  my ($lines, $fhref) = @_;
  $$fhref = *STDOUT unless $$fhref;
  print {$$fhref} $lines;
}

##</code><code>##

my $fh = undef;
printout('some lines1', $fh);
open $fh, '>', 'xxx';
printout('some lines2', $fh);
close $fh;

sub printout {
  my ($lines, $_fh) = @_;
  my $fh = $_fh ? $_fh : *STDOUT;
  print $fh $lines;
}