Use a sort sub-routine. Don't know how you want to sort (you give no example :), but here's a start: 
# sample data
my @filenames = '20001212_20011212_text.asp',
                '20021212_20031212_text.asp',
                '20031212_20041212_image.asp',
                '20011212_20021212_image.asp');

# loop
for (sort { sort_me(); } @filenames) {
  # whatever
  print "$_\n";
}

sub sort_me {

  # grab info from special vars $a and $b
  my ($a_date1,$a_date2) = split '_', $a;
  my ($b_date1,$b_date2) = split '_', $b;

  # then do your sort - eg, to sort by date1 then date2
  # (in the date format I have used above)
  ($a_date1 <=> $b_date1)
  ||
  ($a_date2 <=> $b_date2);
}

[download]

$a and $b are special vars used within a sort. Read this for more info.

Note, this is a rough demo - not gonna test it unless you supply actual data and comparisons... but should give you an idea where to start.

cLive ;-)

Update - just realised you don't need to know type, so have amended code accordingly. later one, removed semi-colon that didn't affect outcome, but still looked out of place...

In reply to Re: How do I sort by certain parts of a filename? by cLive ;-)
in thread How do I sort by certain parts of a filename? by wstarrs

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.