The following works for the range of formats you have given.

use strict;
use warnings;

my %months =
  (
  january =>  'jan',
  february => 'feb',
  march =>    'mar',
  april =>    'apr',
  may =>      'may',
  june =>     'jun',
  july =>     'jul',
  august =>   'aug',
  september =>'sep',
  october =>  'oct',
  november => 'nov',
  december => 'dec',
  );

my %monthCvt =
  (
  jan => 1, feb => 2, mar => 3, apr => 4, may => 5, jun => 6,
  jul => 7, aug => 8, sep => 9, oct => 10, nov => 11, dec => 12,
  );

while (<DATA>)
  {
  chomp;
  $_ = lc $_;

  # Strip leading spaces
  s/\s*//;

  # Normalize month
  s/([a-z]+)/$months{$1} || $1/e;

  # Normalize punctuation
  s|[ -/,.\\]+| |g;

  # Normalize order: month day year
  s|^(\d+)\s([a-z]+)|$2 $1|;

  # Extract fields
  my ($month, $day, $year) = /(\w+) (\d+) (\d+)/;

  # Month to numeric form if required
  $month = $monthCvt{$month} if $month !~ /\d/;

  print "$month/$day/$year\n";
  }

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

Prints:

9/29/2005
6/30/2003
12/15/2005
12/31/2004
12/06/2005
10/19/2005
[download]