$first{$first} = [] if (!$first{$first});

##</code><code>##

#! perl
use strict;
use warnings;

my %names;

while (<DATA>)
{
    my ($first, $last) = split;
    push @{$names{$first}}, $last if $last;
}

my $name_to_find = 'John';
print "$name_to_find --> ", join(', ', @{$names{$name_to_find}}), "\n";

__DATA__
John Marry
John Lea
John Paul
David Patrik
David Sam

##</code><code>##

16:53 >perl 545_SoPW.pl
John --> Marry, Lea, Paul

16:56 >