I got it to do most of what I wanted it to: you reference the members as scalars; you can execute whatever code you want; if you refer to a variable that isn't a member name, it uses the ordinary variable (in fact, if you have a lexical with the same name, it will ignore the member).

But you have to turn off strict, and it definitely pollutes the main namespace (or another namespace, if you litter your code with package statements). So I don't really recommend it, but it can be done..

use warnings;

my %hash = (foo => 'bar', a => 'x');

sub with (\%&) {
  my ($href, $cref) = @_;
  *$_ = \$href->{$_} for keys %$href;
  $cref->();
}

print "$_: $hash{$_}\n" for keys %hash;

my $z = 6;

with %hash,
  sub {
    print "** $foo **!\n";
    $a = 5;
    $z = 'changed lexical';
  }
;

print "$_: $hash{$_}\n" for keys %hash;
print "Z is $z\n";
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Update: got rid of string eval, at least.