I'm thinking that maybe you just want hash arrays? Do you really want to keep duplicates in the array?

my (%last_names, %first_names);
while (<>) {
 my ($last, $first) = split /,/;
 # Instead of undef, you could use '++' if you want to
 # count names e.g.
 # $last_names{$last}++;
 # or treat as an array and push the data '$_' to the hash,
 # e.g.
 # push @{$last_names{$last}}, $_;
 # Or even create a hash of hashes, etc.
 $last_names{$last} = undef;
 $first_names{$first} = undef;
}

print "$_\n" for sort keys %last_names;
#etc.
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