Have solved problem but I dont understand why one lookahead works and another does not.

use strict;
use warnings;
use WWW::Curl::Easy;
my $curl = WWW::Curl::Easy->new;

$curl->setopt(CURLOPT_HEADER,1);
$curl->setopt(CURLOPT_URL, 'http://www.reddit.com/r/wallpapers.rss');
my $response_body;
$curl->setopt(CURLOPT_WRITEDATA,\$response_body);
# Starts the actual request
my $retcode = $curl->perform;
# Looking at the results...
if ($retcode == 0){
  print("Transfer went ok\n\n");
  my $response_code = $curl->getinfo(CURLINFO_HTTP_CODE);
   my @urls=$response_body =~ m{(http://b.thumbs.redditmedia\.com/(?:(
+?!png).)*?\.jpg)}gi;
  $" ="\n\n";
  print "@urls\n";
} else {
  # Error code, type of error, error message
  print("An error happened: $retcode ".$curl->strerror($retcode)." ".$
+curl->errbuf."\n");
}
[download]

Thank you all that helped.

The regex
    (?!pattern).
simply says that whatever  . matches is not the start of whatever  pattern matches (which can be any regex whatsoever). Wrapping this in a non-capturing group
    (?:(?!pattern).)
allows you to quantify the grouped expression in the usual way, so
    (?:(?!pattern).)*?
means "zero or more of anything as long as it doesn't begin a  pattern sequence, with lazy instead of greedy matching". (And I'll go to my grave using "lazy" instead of "non-greedy" as the antonym of "greedy" matching.)

Here's what YAPE::Regex::Explain sez, but I don't think it captures the essence of the concept as clearly:

c:\@Work\Perl\monks>perl -wMstrict -le
"use YAPE::Regex::Explain;
 ;;
 print YAPE::Regex::Explain->new('(?:(?!pattern).)*?')->explain;
"
The regular expression:

(?-imsx:(?:(?!pattern).)*?)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?:                      group, but do not capture (0 or more times
                           (matching the least amount possible)):
----------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
      pattern                  'pattern'
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
    .                        any character except \n
----------------------------------------------------------------------
  )*?                      end of grouping
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
[download]

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