#!/usr/bin/perl  
use strict;  
use warnings FATAL => qw/all/;  
  
my %hash; 
  
open my $ifh, '<', 'input.txt' or die "cannot open: $!";  
my $header = <$ifh>; 
while ( my $line = <$ifh> )  
{  
    chomp $line;  
    my ($w) = split /\t/, $line, 2; 
    $hash{$w} = $line; 
}  
close $ifh;  
 
open my $ofh, '>', 'output.txt' or die "cannot open: $!"; 
print $ofh $header; 
print $ofh "$hash{$_}\n" foreach (sort { $a <=> $b } keys %hash); 
close $ofh;

#!/usr/bin/perl   
use strict;   
use warnings FATAL => qw/all/;   
   
open my $ifh, '<', 'input.txt' or die "cannot open: $!"; 
open my $ofh, '>', 'output.txt' or die "cannot open: $!"; 
 
my $header = <$ifh>; 
print $ofh $header; 
print $ofh map { chomp $_; "$_\n" } sort customsort <$ifh>; 
 
close $ofh; 
close $ifh; 
 
sub customsort 
{ 
    my ($a_w) = split /\t/, $a, 2; 
    my ($b_w) = split /\t/, $b, 2; 
    $a_w <=> $b_w; 
}