$ perl -MData::Dumper -e"%h=(a=>{1..4},b=>{5..8});print Dumper\%h"
$VAR1 = {
          'a' => {
                   '1' => 2,
                   '3' => 4
                 },
          'b' => {
                   '7' => 8,
                   '5' => 6
                 }
        };


##</code><code>##

use strict;
use warnings;
use Data::Dumper;

...

my %hash;
while (<INPUT>) {
   chomp;
   my @line = split /\s+/, $_;
   my ($key1,$key2) = (shift @line, shift @line);
   $hash{$key1} = $hash{$key2} = [@line];
}

print Dumper \%hash;