Greetings all,
Quick and marginally tested solution. you have been warned.

#!/usr/bin/perl -w
use strict;

#set up the test data
my @potential_links = ('[url|desc|target]','[www.test.com|test page]',
+'[www.test.com|test page|new]');

#using the test data
foreach(@potential_links){
  #strip out the braces
  $_ =~ s/\[|\]//g;

  #split the remaining string represented by $_ on the pipe
  my($url, $desc, $target) = split /\|/;

  #ternary to test if $target was set if not print the non-target vers
+ion.
  ($target) ? print qq*<a href="$url" target="$target">$desc</a>\n* : 
+print qq*<a href="$url">$desc</a>\n* ; 
}
exit;
[download]

Is that close to what you are looking for?
-injunjoel