use strict; use warnings;
use feature 'say';

my %result;

for my $line (<DATA>) {
    chomp $line;
    my ( $name, $date, $val ) = split ' ', $line;
    push @{ $result{ $name }{ $date } }, $val;
}

for my $name ( keys %result ) {
    say $name;
    for my $date ( keys %{ $result{ $name } } ) {
        say "\t$date: @{ $result{ $name }{ $date } }";
    }
}

__DATA__
nick 20/5/1950 one
john 18/2/1980 two
nick 19/6/1978 three
nick 20/5/1950 four
nick 12/9/2000 five
john 15/6/1997 six
nick 20/5/1950 seven

##</code><code>##

$ perl 1221691.pl

john
	15/6/1997: six
	18/2/1980: two
nick
	19/6/1978: three
	20/5/1950: one four seven
	12/9/2000: five