I know others have already posted their versions, but here is my attempt at a data-driven implementation. It allows alternative delimiters, but requires that the chosen delimiter is the same throughout the date expression. If you want to validate the incoming date before choosing a format, you should use Date::Manip or something similar.

#!/usr/bin/perl
use strict;
use warnings;
my %Pattern =   ( '(\d{1,2})([-\/.])(\d{1,2})\2(\d{2})' => 'MM%sDD%sRR
+',
                  '(\d{1,2})([-\/.])(\d{1,2})\2(\d{4})' => 'MM%sDD%sYY
+YY',
                  '(\d{4})([-\/.])(\d{1,2})\2(\d{1,2})' => 'YYYY%sMM%s
+DD'
                );

sub ChooseDateFmt($)
{
my $datestr = shift or die;

for my $p (keys %Pattern)
        { return sprintf($Pattern{$p}, $2, $2) if $datestr =~ /^\s*$p\
+s*$/; }
}

while (<DATA>)
        {
        chomp;
        print "Date\t$_\tFormat=" . ChooseDateFmt($_) . "\n";
        }

__DATA__
10/15/2006
2006/11/01
11/01/06
2006.11.01
11-01-06
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Output:

Date    10/15/2006      Format=MM/DD/YYYY
Date    2006/11/01      Format=YYYY/MM/DD
Date    11/01/06        Format=MM/DD/RR
Date    2006.11.01      Format=YYYY.MM.DD
Date    11-01-06        Format=MM-DD-RR
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