First of all, let me say that I certainly don't recommend that you do it this way :)

And yes, I did see your note about not wanting to use external modules....

...but, for the sake of the exercise, and just to prove that it can be done - here it is extending upon my earlier reply using Regexp::Common::time :)

#!/usr/bin/perl -w
use strict;
use Regexp::Common qw(time);

while (<DATA>) {
    print if ($_ =~ $RE{time}{strftime}{-pat =>
        '((%B|%b) %d|%d-%b|%m-%d)[\s\-,]+%Y'});
}

__DATA__
 Sep 29 2005
some stuff
 Jun 30, 2003
some more stuff
 December 15 2005
not a date
 December 31, 2004
 06-Dec-2005
 10-19-2005
[download]

Which gives:

 Sep 29 2005
 Jun 30, 2003
 December 15 2005
 December 31, 2004
 06-Dec-2005
 10-19-2005
[download]

Cheers,
Darren :)