Can you explain ...? I'd like to understand a little bit better ...

You can gain insight yourself into the operation of this (or any) program by inserting print/dump statements at strategic points to see how data is extracted and transformed and how structures are built up:

use Data::Dumper;
...
my %keys;

while(<DATA>)
   {
   push @secondpass, $_;
   $keys{$_}++ for /SATID (\d\d)/g;
   }
print Dumper \%keys;

my @keyorder = sort keys %keys;
print Dumper \@keyorder;

for (@secondpass)
  {
  my %items = reverse /(SATID (\d\d).{27})/g;
  print Dumper \%items;  <STDIN>;
  print join ' ', map { $items{$_} // ' ' x 35 } @keyorder;
  }
...
[download]

The  <STDIN>; after the print statement in
    print Dumper \%items;  <STDIN>;
in the for-loop just pauses output until you hit Enter so you are not overwhelmed by stuff. (NB: My personal favorite structured data dumper is Data::Dump, but Data::Dumper is core. :)