Updated: Added in the forgotten opening brace before the print.

The parent of your node seems to work, while the first response to it doesn't work the same way. I'll break down the one that works :)

perldoc perlrun says:

       -n   causes Perl to assume the following loop around your
            program, which makes it iterate over filename argu-
            ments somewhat like sed -n or awk:

              LINE:
                while (<>) {
                    ...             # your program goes here
                }
[download]

So, with the golfed version: who|perl -ne'/ /,$;{$`}++}{print$_,$/^="*"for%', we can take out what was inside the single quotes and put that together manually:

while(<>){
  / /, $;{$`}++}{print $_, $/ ^= "*" for %
}
[download]

The interesting thing to notice is now this code isn't properly indented, so let's fix that:

while(<>){
  / /, $;{$`}++
}
{
  print $_, $/ ^= "*" for %
}
[download]

What I didn't realize is that apparently -n also apparently adds (?) terminal semicolons (someone correct me if I'm wrong, but with what's happening here certainly seems to be the case), so now it's better re-written as:

while(<>){
  / /, $;{$`}++
}
{
  print $_, $/ ^= "*" for %;
}
[download]

Starting to get clearer. Let's just make it crystal clear:

use English;

while( <ARGV> ){
  m/\s/;
  $hash{$PREMATCH}++;
}
{
  for( %hash ){
    print $_, $INPUT_RECORD_SEPARATOR ^= '*';
  }
}
[download]

All that's left to explain is that:

• $PREMATCH contains the username
• $hash{'username'} counts how many times a user is logged in
• $INPUT_RECORD_SEPARATOR (by default, a newline) when XOR'd with an asterisk becomes a space, such that the first time through the loop (on the first hash key), "\n" ^ "*" is a space, and now that's stored back in $INPUT_RECORD_SEPARATOR. Next time through the loop (the first hash value), a space XOR'd with an asterisk is a newline, and stored back in $INPUT_RECORD_SEPARATOR.

Hence the output:

$username $num_logins
$username $num_logins
[download]

Running it through B::Deparse confirms this:

$ who|perl -MO=Deparse -ne'/ /,$;{$`}++}{print$_,$/^="*"for%'
LINE: while (defined($_ = <ARGV>)) {
    / /, ++$;{$`};
}
{
    foreach $_ (%;) {
        print $_, $/ ^= '*';
    }
}
[download]

s**lil*;  $*=join'',sort split q**;  s;.*;grr; &&s+(.(.)).+$2$1+; $; =
qq-$_-;s,.*,ahc,;$,.=chop for split q,,,reverse;print for($,,$;,$*,$/)
[download]