my @systems = qw(
    this_system_should_be_first
    system1
    ...
    );
my %systems = (
    this_system_should_be_first => 'hostname_to_first_system',
    system1                     => 'hostname_to_system1',
    ...
);
[download]

Too much typing there, use hash slice man (after populating "@systems") ...

my %systems;
@{ @systems } = 
  qw( 
      hostname_to_first_system
      hostname_to_system1
      ...
    ) ;
[download]