$ perl -MO=Deparse,-p -le " print grep( not $foo{$_} , @bar )"
Not enough arguments for grep at -e line 1, near "@bar )
"
-e had compilation errors.
BEGIN { $/ = "\n"; $\ = "\n"; }
print(grep);

$ perl -MO=Deparse,-p -le " print grep( !$foo{$_} , @bar )"
BEGIN { $/ = "\n"; $\ = "\n"; }
print(grep((!$foo{$_}), @bar));
-e syntax OK

$ perl -MO=Deparse,-p -le " print grep( not( $foo{$_}) , @bar )"
BEGIN { $/ = "\n"; $\ = "\n"; }
print(grep((!$foo{$_}), @bar));
-e syntax OK