use strict; 

my %hash = ( a => 1, b => 2, c => 3 );

sub with(\%&) {
  my ($href, $cref) = @_;
  my $cmd = join "\n",
    map sprintf('local $::%s = \\$href->{%s};', $_, $_),
    keys %$href;
  eval $cmd . '$cref->()';
  warn "$@: $cmd\n" if $@;
}

with %hash,
  sub {
    $$a = 'changed';
    print "B=$$b\n";
  }
;

use Data::Dumper;
print Dumper(\%hash);