Certain Perl built-ins, including -s can take an argument or operate on the default variable $_. For example, defined $var versus just defined.

In certain circumstances, perl needs a little help to disambiguate. In this case, it's not sure whether you mean an ill-formed glob:

-s(<1000)
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or

(-s) < 1000
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perl could theoretically look ahead searching for a ">" to disambiguate, but IIRC it only ever looks ahead one token.

Other ways to disambiguate would be to use a different operator which suffers from fewer ambiguities:

grep { 1000 > -s } @ARGV; # or
grep { -s <= 999 } @ARGV;
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or to place parentheses like this:

-s() < 1000
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or include an explicit $_:

-s $_ < 1000
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This is not Windows specific. It could theoretically change between Perl releases, but the oldest and newest versions of Perl I have available on this computer (5.8.9 and 5.16.0) both behave the same with regard to this particular example. So I don't know why your book contains the bad example; perhaps the writer never actually tested it?

So it seems like this must have broken after the book was written. Sounds like a bug to me.

To reinforce tobyink’s answer:

Is it the windows peculiarity which I have here?

No, with

use strict;
use warnings;
use diagnostics;
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I get identical messages under cmd.exe (perl 5, version 16, subversion 0 (v5.16.0) built for MSWin32-x86-multi-thread-64int) and Cygwin (perl, v5.10.1 (*) built for i686-cygwin-thread-multi-64int):

$ perl -c 298_SoPW.pl
Warning: Use of "-s" without parentheses is ambiguous at 298_SoPW.pl l
+ine 36 (#1)
    (S ambiguous) You wrote a unary operator followed by something tha
+t
    looks like a binary operator that could also have been interpreted
+ as a
    term or unary operator.  For instance, if you know that the rand
    function has a default argument of 1.0, and you write

        rand + 5;

    you may THINK you wrote the same thing as

        rand() + 5;

    but in actual fact, you got

        rand(+5);

    So put in parentheses to say what you really mean.

Unterminated <> operator at 298_SoPW.pl line 36 (#2)
    (F) The lexer saw a left angle bracket in a place where it was exp
+ecting
    a term, so it's looking for the corresponding right angle bracket,
+ and
    not finding it.  Chances are you left some needed parentheses out
    earlier in the line, and you really meant a "less than".

Uncaught exception from user code:
        Unterminated <> operator at 298_SoPW.pl line 36.
 at 298_SoPW.pl line 36
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If I search for files bigger than 1000 the parentheses are not necessary.

I take it you mean:

my @selection =  grep { -s > 1000 } @ARGV;
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Here there is no ambiguity, since > doesn’t begin any other operator, as < does.

Update: The following excerpt from -X may also be of (tangential) interest:

These operators are exempt from the "looks like a function rule" described above. That is, an opening parenthesis after the operator does not affect how much of the following code constitutes the argument. Put the opening parentheses before the operator to separate it from code that follows (this applies only to operators with higher precedence than unary operators, of course):

     -s($file) + 1024  # probably wrong; same as -s($file + 1024)
    (-s $file) + 1024  # correct
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Hope that helps,

@ARGV = map{glob} @ARGV;
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It does something analogous to the type of wild-card expansion that *nix shells do:

@ARGV = '*.pl';;

@ARGV = map{glob} @ARGV;;

print for @ARGV;;
1.pl
100GB.pl
1gbdb.pl
24to32.pl
...
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---

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority".

In the absence of evidence, opinion is indistinguishable from prejudice.

RIP Neil Armstrong

This statement allows to use glob in cmd (Win32). Without this line the script if run as:

C:\perl\bin>perl script.pl N:/TEMP/*
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throws the following message:

Use of uninitialized value in numeric lt (<) at script.pl
line 8.
        N:/TMP/*
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I have found this line by BrowserUK some time ago (cannot find the original node now) and I am very grateful for it.

Update: Did not see the post from BrowserUK before I sent my message.

I agree that

 @ARGV = map{glob} @ARGV;
....
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does work, however, I think since "glob" can do the expansion, why not use

my @smaller_than_1000 = grep { -s $_ < 1000 } glob("@ARGV");
....
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see glob for detail.
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

Basic debugging checklist, B::Deparse, its for everybody