I don't believe it is leaking memory? Here's a quote from the perldoc for WWW::Mechanize.

Mech also stores a history of the URLs you've visited, which can be queried and revisited.

Also, here's the output I get from your program.

agent size = 123086
agent size = 245249
agent size = 367383
agent size = 489517
agent size = 611651

This shows me that the size is going up by an almost constant size each loop. I then modified your program to use Data::Dumper and spit out the results after five GETs. Here's the code.

#!/usr/bin/perl -w
                                                                      
+                                           
use strict;
use WWW::Mechanize;
use Devel::Size qw(size total_size);
use Data::Dumper;
                                                                      
+                                           
my $agent = WWW::Mechanize->new();
                                                                      
+                                           
for(my $i = 0; $i < 5; $i++) {
  $agent->get(qq(http://www.yahoo.com));
  print "agent size = ".total_size($agent)."\n";
}
                                                                      
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print Dumper($agent);
[download]

Looking at the output, there is a data structure that is part of the module called the page_stack. I'm guessing, that the implementation of the back() method uses the page_stack. That way, rather than re-requesting the pages, the pages are just reloaded from memory. I don't think this is a leak. This is just appears to be the functionality of the module.