For Perl lacking threads support (i.e. not compiled with threads enabled), one may run threads-like code using MCE::Hobo. Here is similar code using MCE::Hobo and MCE::Shared.

#!/usr/bin/env perl

use strict;
use warnings;

use MCE::Hobo;
use MCE::Shared;
use IO::Socket;

my $nthreads = 20;

my $in_file2 = 'rang.txt';

my $work_q   = MCE::Shared->queue;
my $result_q = MCE::Shared->queue;

sub ip_checker {
    while ( my $ip = $work_q->dequeue ) {
        chomp($ip);
        my $host = IO::Socket::INET->new(
            PeerAddr => $ip,
            PeerPort => 80,
            proto    => 'tcp',
            Timeout  => 1
        );
        if ( defined $host ) {
            $result_q->enqueue($ip);
        }
    }
}

sub file_writer {
    open( my $output_fh, ">>", "port.txt" ) or die $!;
    while ( my $ip = $result_q->dequeue ) {
        print {$output_fh} "$ip\n";
    }
    close($output_fh);
}

my @workers;
for ( 1 .. $nthreads ) {
    push( @workers, MCE::Hobo->create( \&ip_checker ) );
}
my $writer = MCE::Hobo->create( \&file_writer );

open( my $dat, "<", $in_file2 ) or die $!;
$work_q->enqueue(<$dat>);
close($dat);
$work_q->end;

foreach my $thr (@workers) {
    $thr->join();
}

$result_q->end;
$writer->join();
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MCE::Child is also threads-like, but does not require MCE::Shared to run. The complementary module for exchanging data is MCE::Channel.

#!/usr/bin/env perl

use strict;
use warnings;

use MCE::Child;
use MCE::Channel;
use IO::Socket;

my $nthreads = 20;

my $in_file2 = 'rang.txt';

my $work_q   = MCE::Channel->new;
my $result_q = MCE::Channel->new;

sub ip_checker {
    while ( my $ip = $work_q->dequeue ) {
        chomp($ip);
        my $host = IO::Socket::INET->new(
            PeerAddr => $ip,
            PeerPort => 80,
            proto    => 'tcp',
            Timeout  => 1
        );
        if ( defined $host ) {
            $result_q->enqueue($ip);
        }
    }
}

sub file_writer {
    open( my $output_fh, ">>", "port.txt" ) or die $!;
    while ( my $ip = $result_q->dequeue ) {
        print {$output_fh} "$ip\n";
    }
    close($output_fh);
}

my @workers;
for ( 1 .. $nthreads ) {
    push( @workers, MCE::Child->create( \&ip_checker ) );
}
my $writer = MCE::Child->create( \&file_writer );

open( my $dat, "<", $in_file2 ) or die $!;
$work_q->enqueue(<$dat>);
close($dat);
$work_q->end;

foreach my $thr (@workers) {
    $thr->join();
}

$result_q->end;
$writer->join();
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The modules are interchangeable. One may use threads with MCE::Shared or MCE::Channel. Ditto for using Parallel::ForkManager with MCE::Shared or MCE::Channel. Here, we have a shared array.

#!/usr/bin/env perl

use strict;
use warnings;

use Parallel::ForkManager;
use MCE::Shared;
use IO::Socket;

my $in_file2 = 'rang.txt';
open( my $input,  "<", $in_file2 )  or die $!;
my $result = MCE::Shared->array;

my $manager = Parallel::ForkManager->new(20);
foreach my $ip (<$input>) {
    $manager->start and next;

    MCE::Shared->init(); # Optional
      # This is called automatically for threads, MCE, MCE::Hobo,
      # and MCE::Child. Calling MCE::Shared->init assigns a data
      # channel in a round-robin fashion to ForkManager workers.
      # Omitting init has the effect of 1 data channel versus 10.

    chomp($ip);
    my $host = IO::Socket::INET->new(
        PeerAddr => $ip,
        PeerPort => 80,
        proto    => 'tcp',
        Timeout  => 1
    );
    if ( defined $host ) {
        $result->push($ip);
    }
    $manager->finish;
}
$manager->wait_all_children;
close($input);

# move shared array to unshared array reference
$result = $result->destroy({ unbless => 1 });

open( my $output, ">", "port.txt" ) or die $!;
print {$output} "$_\n" for @{ $result };
close($output);
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The shared array resides under the shared-manager process. This is where shared data resides. Upon completion, one can export the data or export-destroy via $result->destroy. The object is MCE::Shared::Array or an array reference [] for unbless => 1. Not exporting will involve making individual trips per each item. One can call $result->values which exports the values as well.

Here is MCE::Flow where workers read from the input file directly. MCE->gather is called to append to an array.

#!/usr/bin/env perl

use strict;
use warnings;

use MCE::Flow;
use IO::Socket;

my $nthreads = 20;
my $in_file2 = 'rang.txt';

sub ip_checker {
    my $ip = $_;
    chomp($ip);
    my $host = IO::Socket::INET->new(
        PeerAddr => $ip,
        PeerPort => 80,
        proto    => 'tcp',
        Timeout  => 1
    );
    if ( defined $host ) {
        MCE->gather($ip);
    }
}

my @result = MCE::Flow->run_file(
    { chunk_size => 1, max_workers => $nthreads },
    \&ip_checker,
    $in_file2,
);

MCE::Flow->finish();

open( my $output, ">", "port.txt" ) or die $!;
print {$output} "$_\n" for @result;
close($output);
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For this one, workers write serially to the output file via MCE->print.

use strict;
use warnings;

use MCE::Flow;
use IO::Socket;

my $nthreads = 20;
my $in_file2 = 'rang.txt';
open( my $output, ">", "port.txt" ) or die $!;

sub ip_checker {
    my $ip = $_;
    chomp($ip);
    my $host = IO::Socket::INET->new(
        PeerAddr => $ip,
        PeerPort => 80,
        proto    => 'tcp',
        Timeout  => 1
    );
    if ( defined $host ) {
        MCE->print($output, "$ip\n");
    }
}

MCE::Flow->run_file(
    { chunk_size => 1, max_workers => $nthreads },
    \&ip_checker,
    $in_file2,
);

MCE::Flow->finish();
close($output);
[download]

Perl provides various ways to concur parallel processing. There is Parallel::ForkManager. For threads-like API, there is MCE::Child and MCE::Hobo. MCE itself provides chunking capabilities. It has sugar syntax to gather into an array or hash. For output, MCE->print, MCE->printf, and MCE->say write serially to a file handle (default STDOUT if omitted).

Basically, no problem if the Perl binary lacks threads support.