#!/usr/bin/perl -w
use strict;
use threads;
use threads::shared;
use Cache::File::Heap;

my $heap = Cache::File::Heap->new('frontier');
my $heap_lock : shared = 0; 

...

sub go {#crawling thread's control flow
       ....
       #xtracted best promising url
       {
           lock $heap_lock;
           my($value, $url) = $heap->extract_minimum;
       }

       ...
       #after downloading and extract hyperlinks
       {
           lock $heap_lock;
           $heap->add($value, $url) for 
        }
        ...
}

my @threads;
for(1..10) {
    push @threads, threads->new(\&go);
}
for(@threads) {
    $_->join;
}