Here's a slight variation on the three-fields-versus-four approach: if there are four fields, the fourth must be 'Y', which is true. (I use unshift instead of push for the  @streams array because that gives the ordering of elements shown in the OPed example, which I take to be specificatory.)

c:\@Work\Perl>perl -wMstrict -MData::Dump -le
"my (@streams, @spaces);
 ;;
 for my $result (
   'ws.abc.dsfsfsfwerfrwef std 13232',
   'ws.dfsdferferfregregrgr std 99868 Y',
   'ws.abc.asddwefewfewfewf std 45345',
   'fwfrwfrefergegregerefds std 36754 Y',
   'ws.abc.fferfergregrfreg std 45435',
   ) {
   my ($first, undef, undef, $fourth) = split m{ \s+ }xms, $result;
   if ($fourth) {
     unshift @streams, $first;
     }
   else {
     push @spaces, $first;
     }
   }
 ;;
 dd \@streams;
 dd \@spaces;
"
["fwfrwfrefergegregerefds", "ws.dfsdferferfregregrgr"]

[
  "ws.abc.dsfsfsfwerfrwef",
  "ws.abc.asddwefewfewfewf",
  "ws.abc.fferfergregrfreg",
]
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Update: One could also use the simple test  @fields == 4 if split-ing to an array.