With that said and given your example data - I would:

#!/usr/bin/perl -w
use strict;
my %seen;
my @data;
open (INPUT,"file") or die "Error accessing file : $!";
while (<INPUT>) {
    chomp;
    my ($col1, $col2) = split /\t/;
    if (exists $seen{$col1}) {
        $data[$seen{$col1} - 1] .= " $col2";
    }
    else {
        $seen{$col1} = push @data , "$col1\t$col2";
    }
}
print "$_\n" foreach(@data);
[download]

Cheers - L~R

Update: Completely re-wrote code to be unique as my original solution closely mirrored others