A couple of points:

1. Don’t use first as the name of both the hash and the scalar! Yes, Perl can easily disambiguate them, but why make things harder (for humans) to read and comprehend than they need to be?

2. The line:

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

   isn’t needed — autovivification will take care of that for you automatically.

#! 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
[download]

Output:

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

16:56 >
[download]

Hope that helps,