# thezip's method:
$fn = sprintf "%s.%04d-%02d-%02d-%02d%02d%02d.ext",
              $base_name,
              (localtime)[5,4,3,2,1,0];

##</code><code>##

# my method (from CB):
$fn = '';
{
  open my $fh, '>', \$fn;
  print $fh $base_name, '.';
  print $fh strftime("%F-%T");
}