Since the OP mentions that the "Y" will be at the end, why not use a [ -1 ] subscript to avoid the "defined" complication? You could also use a single push with the conditional and target arrays in a ternary. Some may find this less readable than the if ... else ... but I prefer it for simple "push to this or that depending" situations.

$ perl -Mstrict -Mwarnings -E '
open my $inFH, q{<}, \ <<EOD or die $!;
ws.abc.dsfsfsfwerfrwef std 13232
ws.dfsdferferfregregrgr std 99868 Y
ws.abc.asddwefewfewfewf std 45345
fwfrwfrefergegregerefds std 36754 Y
ws.abc.fferfergregrfreg std 45435 
EOD

my @streams;
my @spaces;

while ( <$inFH> )
{
    my @flds = split;
    push @{ $flds[ -1 ] eq q{Y} ? \ @streams : \ @spaces },
       $flds[ 0 ];
}

say qq{@streams};
say qq{@spaces};'
ws.dfsdferferfregregrgr fwfrwfrefergegregerefds
ws.abc.dsfsfsfwerfrwef ws.abc.asddwefewfewfewf ws.abc.fferfergregrfreg
$
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I hope this is of interest.

Since the OP mentions that the "Y" will be at the end, why not use a [ -1 ] subscript to avoid the "defined" complication?

That works, but what if you just so happen to have a "Y" in the third field? The OP's example lines only have numbers, granted, but I didn't want to assume too much about how his/her data is structured.

You could also use a single push with the conditional and target arrays in a ternary. Some may find this less readable than the if ... else ... but I prefer it for simple "push to this or that depending" situations.

Ah, yes, that's a good point. I was trying that, actually; it didn't work ("Not an ARRAY reference at ..."), but it didn't occur to me to wrap the whole shebang in a @{ }. OTOH, between that and the need to take references to the arrays you want to push to, it is indeed on the cryptic side.

Nevertheless, I learned something new. Thank you for enlightening me, brother!

..That works, but what if you just so happen to have a "Y" in the third field?..

For the lines without the Y there would be no ^*four^th field counting from 0, check or count it.
And even if you decide to include Y in the those lines last field if($fields[-1] eq 'Y'){.. would still work.

---

*Updated this in light of the discuss that follows! Which has nothing to do with the level of correctness as some may presume.
If you tell me, I'll forget.
If you show me, I'll remember.
if you involve me, I'll understand.
--- Author unknown to me

The push reads better without the obfuscating ref/deref stuff:

push (($flds[-1] eq 'Y' ? @streams : @spaces), $flds[0]);
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Update: Ok, so that was a fail - struck

Perl is the programming world's equivalent of English

Reads better perhaps, but doesn't actually work. First argument to push must be something starting with the  @ sigil or, with Perl 5.14+, a reference to an unblessed array.

c:\@Work\Perl>perl -wMstrict -le
"print $];
 ;;
 my (@streams, @spaces);
 my $y = 'Y';
 push (($y eq 'Y' ? @streams : @spaces), 'Z');
 print qq{(@streams) (@spaces)};
"
5.014004
Not an ARRAY reference at -e line 1.

c:\@Work\Perl>perl -wMstrict -le
"print $];
 ;;
 my (@streams, @spaces);
 my $y = 'Y';
 push (($y eq 'Y' ? \@streams : \@spaces), 'Z');
 print qq{(@streams) (@spaces)};
"
5.014004
(Z) ()
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