push (@{$ttys{$user}}, $tty );

##</code><code>##

my %ttys = ();

open(WHO, "who|");

while (<WHO>)
{
($user, $tty) = split;
push (@{$ttys{$user}}, $tty ); <--- WHAT ???
}

foreach $user (sort keys %ttys)
{
print "$user: @{$ttys{$user}}\n";
}