You can read the long answer under perlman:perlref.

For your specific example, this should work as follows (untested) :

  # ... your stuff here, to load $2 and %one with data
  push $users{$2}, \%one;

  ...

  my $user = $users{$key};
  foreach $value ("pid", "cmd", "mem") {
    print %$user{$value}, " ";
  };
[download]