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]

                Memory fault   --   brain fried

In reply to Re: How to map data from one column based on another column in perl by NetWallah
in thread How to map data from one column based on another column in perl by waekit

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