I find that removing the autovivication can make the code easier to understand (and I wish there was a way to force this):

my %ttys= ();

open( WHO, "who|" )
  or  die "Can't fork to read from who: $!";
while( <WHO> ) {
    my( $user, $tty )= split;
    if(  ! exists $ttys{$user}  ) {
        $ttys{$user}= [$tty];
    } else {
        push @{$ttys{$user}}, $tty;
    }
}
foreach my $user ( sort keys %ttys ) {
    print "$user: @{$ttys{$user}}\n";
}
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The if shows a bit of what happens under the covers in the previous example.

        - tye (but my friends call me "Tye")