in reply to How to map data from one column based on another column in perl

Here is some idiomatic code to help get you to where I think you are headed: 
#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;
    
my $filename = 'pedigree_proband_testfile.txt';
my %n = (birth_year=> 13,  motherID=> 4,  individualID=> 2, gender => 
+5);
my %mominfo;

open my $f, "<", $filename or die "ERROR: Cannot open $filename:$!";

while (<$f>){
  chomp;
  my @c = split /\t/, $_; 
  my $currentMomID = $c[ $n{motherID} ]  || $c[ $n{individualID} ] or 
+die "No Mother ID in record nbr $.=$_;";
  my $currentMom = $mominfo{ $currentMomID } ||= {}; # Create an empty
+ mom if she does not exist
  if ($currentMomID == $c[ $n{individualID} ]){
    print "This is Mom ($currentMomID)'s main record: $_\n";
    $currentMom->{birth_year} = $c[ $n{birth_year} ];
  }else{
    push @{ $currentMom->{CHILDREN} }, {ID=>  $c[ $n{individualID} ] ,
+ 
                                        birth_year =>  $c[ $n{birth_ye
+ar} ],
                                        gender => $c[ $n{gender} ]};
  }
}

print Dumper \%mominfo;

[download]
Output: 
This is Mom (2)'s main record: BC000    0    2            M           
+                           999
$VAR1 = {
          '3' => {
                   'CHILDREN' => [
                                   {
                                     'gender' => 'F',
                                     'birth_year' => '51',
                                     'ID' => 1
                                   },
                                   {
                                     'ID' => 4,
                                     'birth_year' => '  50',
                                     'gender' => 'M'
                                   },
                                   {
                                     'gender' => 'F',
                                     'birth_year' => '47',
                                     'ID' => 5
                                   },
                                   {
                                     'birth_year' => undef,
                                     'gender' => 'F',
                                     'ID' => 6
                                   },
                                   {
                                     'gender' => 'M',
                                     'birth_year' => '',
                                     'ID' => 7
                                   },
                                   {
                                     'gender' => 'F',
                                     'birth_year' => '42',
                                     'ID' => 8
                                   },
                                   {
                                     'ID' => 9,
                                     'birth_year' => '39',
                                     'gender' => 'F'
                                   },
                                   {
                                     'birth_year' => '35',
                                     'gender' => 'F',
                                     'ID' => 10
                                   }
                                 ]
                 },
          '8' => {
                   'CHILDREN' => [
                                   {
                                     'ID' => 11,
                                     'gender' => 'M',
                                     'birth_year' => '  11'
                                   },
                                   {
                                     'birth_year' => '  9',
                                     'gender' => 'F',
                                     'ID' => 12
                                   }
                                 ]
                 },
          '2' => {
                   'birth_year' => ''
                 },
          '36' => {
                    'CHILDREN' => [
                                    {
                                      'ID' => 3,
                                      'gender' => 'F',
                                      'birth_year' => '  65'
                                    }
                                  ]
                  }
        };

[download]

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