You will find this a frequent code pattern, young padewan.

Here is a self-contained solution that demonstrates the idioms.

This code uses "Autovivification" to avoid the issue you experienced - i.e. the same code handles both the case where the key is absent, and when present. Read all about it in perlref.

use strict;
use warnings;

my %team;

while (defined (my $line=<DATA>)){
    chomp $line; # Zap the newline
    my ($name,$affiliation) = split /\t/,$line;
    next unless $affiliation; # Avoid empty lines
    push @{ $team{$affiliation} }, $name;
}

# Print the results
for my $aff (sort keys %team){
    print $aff, " :\t",
          join(", ", sort @{ $team{$aff} }) , "\n";
}


__DATA__
Obi Wan    Jedi
Yoda    Jedi
Count Dooku    Sith
Darth Vader    Sith
Luke Skywalker    Jedi
[download]

OUTPUT:

Jedi :    Luke Skywalker, Obi Wan, Yoda
Sith :    Count Dooku, Darth Vader
[download]