Venerable Monks, I came across this error recently and was wondering if someone could explain it to me: 
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)"

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From perldoc:
Unary "not" returns the logical negation of the expression to its right. It's the equivalent of "!" except for the very low precedence.

In reply to Logical Not and arguments to grep by tangent

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