You could do something like the following (using ENV rather than foo):

use strict;
use warnings;
use Data::Dump::Streamer;

my %foo = (a => 'b', c => 'd');
my @stack;

my %newFoo = (x => 'b', y => 'd');

push @stack, {%foo};
%foo = %newFoo;
print "Using \%newFoo\n";
Dump (\%foo);

# do stuff with temporary foo

%foo = %{pop @stack};
print "restored original \%foo\n";
Dump (\%foo);
[download]

Prints:

Using %newFoo
$HASH1 = {
           x => 'b',
           y => 'd'
         };
restored original %foo
$HASH1 = {
           a => 'b',
           c => 'd'
         };
[download]