The "<" in the grep block

[vagabonding electron]

Dear Monks,
doing some exercises from the Alpaca book I noticed a strange behavior of the grep operator.
The exercise was to filter the files smaller than 1000 bytes.
This code (if called with C:\perl\bin>perl script.pl N:/TEMP/*):

#!/usr/bin/perl -l
use strict;
use warnings;
@ARGV = map{glob} @ARGV;
my @selection =  grep { -s < 1000 } @ARGV;
print for @selection;

produces the following error message:

Warning: Use of "-s" without parentheses is ambiguous at script.pl lin
+e 7.
Unterminated <> operator at script.pl line 7.

If I use the parenthesis it runs ok:

#!/usr/bin/perl -l
use strict;
use warnings;
@ARGV = map{glob} @ARGV;
my @selection =  grep { (-s) < 1000 } @ARGV;
print for @selection;

If I search for files bigger than 1000 the parentheses are not necessary.
It seems that perl interprets the less than sign as a part of a diamond operator.
In the answers part of the book however there is a line exact as my original line without parentheses:

my @smaller_than_1000 = grep { -s < 1000 } @ARGV;

Is it the windows peculiarity which I have here?
Thanks in advance!
VE