my $str = `who`;

##</code><code>##

foreach my $line (split /\n/,$str) {
    my ($user, $remainder) = split /\s/,$_,2;
    $hash{$user}++;  # just increment the value
}

##</code><code>##

open(PIPE,"who |") or die "Can't run 'who': $!\n";
while(<PIPE>) {
    my ($user, $remainder) = split;  # implicit split splits $_ on /\s+/
    $hash{$user}++;

##</code><code>##

# sort by user
foreach my $user(sort keys %hash) {
    print "$user logged in $hash{$user} times.\n";
}
# sort by times users are logged in
foreach my $user(sort {$hash{$a} <=> $hash{$b}} keys %hash) {
    print "$user logged in $hash{$user} times.\n";
}