Regexp::Common::time would seem to be made to measure for this task:

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

while (<DATA>) {
    print if ($_ =~ $RE{time}{strftime}{-pat => '%B %d %Y'});
}

__DATA__
December 15 2005

some line

Flat File Release 100.0

someline
[download]

Which prints:

December 15 2005
[download]

Cheers,
Darren :)