#!/usr/bin/env perl
use strict;
use warnings;
use IPC::Open3;

my %ttys;
open3( undef, *HIS_OUT, undef, "/usr/bin/who" );
while (<HIS_OUT>) {
    my ( $user, $tty ) = split;
    push @{ $ttys{$user} }, $tty;
}

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