You'll probably find Parallel::ForkManager useful.

use Parallel::ForkManager qw( );

use constant MAX_CHILDREN => 3;

{
   my $mech = WWW::Mechanize->new(timeout => 90);

   my @urllist = ...;

   my $pm = Parallel::ForkManager->new(MAX_CHILDREN);
   foreach my $url (@urllist) {
      # Forks and returns the pid for the child.
      my $pid = $pm->start() and next; 

      $mech->get($url);
      print $mech->content()->as_HTML();

      # Exit child.
      $pm->finish();
   }
}
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