Here's another option:

use Modern::Perl;
use Data::Dumper;

my %rep;

map {my @x = split; push @{ $rep{ $x[0] } }, $x[1]} grep /\S/, <DATA>;

say Dumper \%rep;

__DATA__
a b

c d

a d

c a

f g

f h
[download]

Output:

$VAR1 = {
          'c' => [
                   'd',
                   'a'
                 ],
          'a' => [
                   'b',
                   'd'
                 ],
          'f' => [
                   'g',
                   'h'
                 ]
        };
[download]

If there are no blank lines in your data set, you can omit the grep /\S/, in the script.

Hope this helps!