You should listen to the other monks who directed you to modules. But I wanted to take a stab at doing it in a regex. This code seems to work: 
#!perl  -w
use strict;

for my $y ( 1753 .. 9999 )
{
for my $m ( 1 .. 12 )
{
for my $d ( 1 .. 31 )
{
my $date = sprintf "%04d/%02d/%02d", $y, $m, $d;

if ( $date !~ 
m/^
########################
# Year 
([2-9]\d{3}|1[89]\d\d|17[6-9]\d|175[3-9])
\/ #####################
# Month
(0[1-9]|1[0-2])
\/ #####################
# Day
(
0[1-9]|1\d|2[0-8]|  # 01 - 28
(?<=(?:0[13578]|10|12)\/)(?:29|3[01])| #  to 31
(?<=(?:0[469]|11)\/)(?:29|30)| #  to 30
(?<=(?:
    (?:2[048]|3[26]|4[048]|5[26]|6[048]|7[26]|8[048]|9[26])00|
    \d\d(?:0[48]|1[26]|2[048]|3[26]|4[048]|5[26]|6[048]|7[26]|8[048]|9
+[26])\/02\/)
    )(?:29) # Leap year
)
########################
$/x 
)
{
print "$date is invalid\n";
}
# Else $1 == year, $2 == month, $3 == day
}}}

[download]
Of course, different countries switched to the Gregorian calendar at different dates, so you really need a module to get it right. My favorite tome on the topic is "Calendrical Calculations" by Edward M Reingold and Nachum Dershowitz.

Update: I realized that I made a mistake listing leap-years. I've now fixed that, but it further demonstrates why a module is better.

In reply to Re: regular expression help by Adam
in thread regular expression help by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.