I don't get it :-( I tried:

  use strict ;
  use warnings ;

  try() ;        #     none: 1
  try(undef) ;   #    undef: 1
  try(-1) ;      #       -1: 0
  try(0) ;       #        0: 0
  try(1) ;       #        1: 1

  sub try {
    my ($s) = @_ ;

    my $r = validate_max_size(@_) ;

    if    (@_ == 0)      { $s = "none" ;  }
    elsif (!defined($s)) { $s = "undef" ; } ;

    printf "%8s: %d\n", $s, $r+0 ;
  } ;

  sub validate_max_size {
    my $val = shift;

    ! defined $val || $val > 0;
  }
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on perl 5.10.0, and got no warnings...

I don't know if there's an ointment for bracket-alergy as extreme as this... I would write:

  !defined($val) || ($val > 0) ;
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'cos I'm comfortable with unaries binding tighter than binaries... but the effect is the same as what you writ.

Wait a tick... this gives the error you mentioned:

  try('undef') ;  # Argument "undef" isn't numeric in numeric gt (>) .
+..
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could it be that somebody, somewhere, is mapping undef to 'undef' ?