use strict;
use warnings;
use LWP::UserAgent;

my @pages = ('http://www.perlmonks.org','http://perldoc.org');

my $ua = LWP::UserAgent->new;
# the line count is global
my $read_lines=1;

foreach my $url (@pages){
        my $response = $ua->get($url, ':content_cb'=>\&head_only);
}

sub head_only{
    my ($data,$resp,$protocol) = @_;
    my @lines = split "\n", $data;
    foreach my $line (@lines){
        if ($read_lines == 31){
           # reset the line count 
           $read_lines = 1;
           print +("=" x 70),"\n";
           # die inside this callback interrupt the request, not the program!!
           # see LWP::UserAgent docs
           die;
        }
        else{
            print "line $read_lines: $line\n"
        }
        $read_lines++;
    }
}