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";