many variations but at glance it seems to me that this is missing: use a hash and put vualues into an array (choroba suggested this..).

Being an opportunity to produce a oneliner, I cannot resist to :)

cat uniqfirst.txt

nick 5
nick 10
nick 20
john 78
erik 9
erik 12

perl -lanE "push@{$r{$F[0]}},$F[1]}{say join' ',$_,$r{$_}[0]for keys%r
+" uniqfirst.txt

erik 9
nick 5
john 78


perl -MO=Deparse -lanE "push@{$r{$F[0]}},$F[1]}{say join' ',$_,$r{$_}[
+0]for keys%r"

BEGIN { $/ = "\n"; $\ = "\n"; }
use feature 'current_sub', 'evalbytes', 'fc', 'postderef_qq', 'say', '
+state', 'switch', 'unicode_strings', 'unicode_eval';
LINE: while (defined($_ = readline ARGV)) {
    chomp $_;
    our @F = split(' ', $_, 0);
    push @{$r{$F[0]};}, $F[1];
}
{
    say join(' ', $_, $r{$_}[0]) foreach (keys %r);
}
-e syntax OK
[download]

L*