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

my %data;
while (<DATA>) {
    my ($k, $v) = split;
    push @{ $data{$k} }, $v;
}
print Dumper(\%data);

__DATA__
k1 5
k2 3
k1 7

##</code><code>##

$VAR1 = {
          'k2' => [
                    '3'
                  ],
          'k1' => [
                    '5',
                    '7'
                  ]
        };