#!/usr/bin/perl
#
use strict;
use warnings;
use Data::Dumper;

my %hash1;
my %hash2;
foreach my $line (<DATA>) {
    my ($key, $value) = split(/\s+/, $line);
    push(@{$hash1{$key}}, $value);
    push(@{$hash2{$value}}, $key);
}

print Dumper(\%hash1);
print Dumper(\%hash2);

print "Values for key a are @{$hash1{a}}\n";
print "Keys with value b are @{$hash2{b}}\n";
print "Keys with value h are @{$hash2{h}}\n";

__DATA__
key value
a   b
a   c
a   d
e   f
g   h
g   i
j   h

##</code><code>##

$VAR1 = {
          'e' => [
                   'f'
                 ],
          'a' => [
                   'b',
                   'c',
                   'd'
                 ],
          'g' => [
                   'h',
                   'i'
                 ],
          'j' => [
                   'h'
                 ],
          'key' => [
                     'value'
                   ]
        };
$VAR1 = {
          'c' => [
                   'a'
                 ],
          'h' => [
                   'g',
                   'j'
                 ],
          'b' => [
                   'a'
                 ],
          'value' => [
                       'key'
                     ],
          'd' => [
                   'a'
                 ],
          'f' => [
                   'e'
                 ],
          'i' => [
                   'g'
                 ]
        };
Values for key a are b c d
Keys with value b are a
Keys with value h are g j