use strict;
use warnings;

my %hash = (abc=>1,def=>1);
my @array = qw/abc def ghi/;
my @ary1 = grep{ ! $hash{$_} } @array;    # OK
my @ary2 = grep( ! $hash{$_}, @array);    # OK
my @ary3 = grep{ not $hash{$_} } @array;  # OK
my @ary4 = grep( not $hash{$_}, @array);  # NOT OK

Errors from fourth statement:
Useless use of hash element in void context
Not enough arguments for grep near "@array)"