Maximum set of unrelated individuals

#!/usr/bin/perl
use strict;
use warnings;

## This program is to ;
## Date : 2015.12.31
## Programmer : yekaixiong

## ^ is not a symbol in perl, rather I should use **
die "Usage:<input of UKB rel file><output of ind to remove>" unless @ARGV == 2;


open IA, "$ARGV[0]"  || die "Can't open IA: $!\n";
open OA, ">$ARGV[1]" || die "OA:$!\n";
<IA>;
my %hash;
my %ind;
my $num_pairs = 0;
while (<IA>) {
    chomp;
    my @tmp = split;
    $hash{$tmp[0]}->{$tmp[1]} = 1;
    $ind{$tmp[0]}++;
    $ind{$tmp[1]}++;
    $num_pairs++;
}
close IA;
print "Total number of related pairs: $num_pairs\n";
my %removed;
my @sorted_ind = sort {$ind{$b} <=> $ind{$a}} keys %ind;
unless(defined $sorted_ind[0]){
    exit;
}
my $rmvind     = $sorted_ind[0];
my $rmpair     = $ind{$rmvind};

while($num_pairs != 0){
    $removed{$rmvind} = $rmpair;
    print "Removed individual $rmvind, involving in $rmpair pairs\n";
    $num_pairs = 0;
    my %new_ind;
    foreach my $k1 (keys %hash){
        delete $hash{$k1} if (defined $removed{$k1});
        foreach my $k2 (keys %{$hash{$k1}}){
            if (defined $removed{$k2}){
                delete $hash{$k1}->{$k2};
            }else{
                $num_pairs++;
                $new_ind{$k1}++;
                $new_ind{$k2}++;
            }
        }
    }
    print "Current number of related pairs: $num_pairs\n";
    if($num_pairs > 0){
        my @new_sorted_ind = sort {$new_ind{$b} <=> $new_ind{$a}} keys %new_ind;
        $rmvind = $new_sorted_ind[0];
        $rmpair = $new_ind{$new_sorted_ind[0]};
    }
}

foreach my $k (keys %removed){
    print OA "$k\n";
}
close OA;