use Time::localtime;

sub timestamp {
  my $t = localtime;
  return sprintf( "%04d-%02d-%02d_%02d-%02d-%02d",
                  $t->year + 1900, $t->mon + 1, $t->mday,
                  $t->hour, $t->min, $t->sec );
}

print '[' . timestamp() . ']: my message'. "\n";

##</code><code>##

[2011-01-08_11-48-29]: my message