Your code is broken in many ways, but two things stand out.

1. You are using (copies) of the same instance of Net::Ping in many threads. Not a good idea, so I've moved its creation inside the thread sub.
2. You are closing the Net::Ping instance, which I've disabled.

This now "works", though your definition of what constitutes a "valid address" is pretty arbitrary:

#! /usr/bin/perl
# to check ping from a file of hostnames using threads

#use strict;
use warnings;
use threads;
use threads::shared;
use Net::Ping;
use Time::HiRes;

my $timeout = 5;

print "\nEnter the host file name:";
chomp (my $host_file = <STDIN>);
open (HOSTFILE, "<", $host_file) or die "\nOpen of $host_file failed. 
+$!";
my @host_array = <HOSTFILE>;
close (HOSTFILE);

print"\nEnter the output file name:";
chomp(my $out_file = <STDIN>);
open (OUTFILE, ">", $out_file) or die "\n Open of $out_file failed.$!"
+;


my @threads;

foreach $addr(@host_array){

    chomp($addr);

    my $thr = threads->new(\&sub_ping,$addr);
    sleep(1);
    push(@threads,$thr);
}

foreach (@threads){
    my $val = $_->join();
    print "Trying to ping $val complete\n";
}

sub sub_ping {
    my $host= shift; #to get $ARGV[0]

    my $ping_handle = Net::Ping->new("icmp",$timeout) or die "\n ping 
+unsuccessful. $!";;
    $ping_handle->hires();

    if (($host =~ m([a-zA-Z]+\.\w+\.\w+)) || ($host =~ m(\d{1,3}\.\d{1
+,3}\.\d{1,3}\.\d{1,3}))) {
        ($ret, $lat, $ip) = ($ping_handle->ping($host));

        if ($ret) {
            printf "$host \($ip\) responds to pings at %.2f msec.\n", 
+($lat//0)*1000;
            printf OUTFILE ("$host \($ip\) responds to pings at %.2f m
+sec.\n",($lat//0)*1000);
        }
        else {
            printf "$host is NOT responding to pings. Time Taken %.2f 
+msec\n",($lat//0)*1000;
            printf OUTFILE ("$host is NOT responding to pings. Time ta
+ken %.2f msec\n",($lat//0)*1000);
        }
        #$ping_handle->close();
    }
    else {
        print "$host is NOT a valid address.\n";
        print OUTFILE ("$host is NOT a valid address.\n");
    }
    return $host;
}
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