use warnings; 
use strict;

sub a       { "unchanged" };
sub print_a { print a(), "\n" };

##</code><code>##

no strict 'refs';
no warnings 'redefine';

{
    local *a = sub { "changed" };
    print_a();  # prints "changed"
}

##</code><code>##

print_a();      # prints "unchanged"

##</code><code>##

sub localize_a_and_call_fn(&@) {
    my ($fn, @args) = @_;
    local *a = sub { "changed" };
    $fn->(@args);
}

##</code><code>##

localize_a_and_call_fn( \&print_a );
# prints "changed"

localize_a_and_call_fn {
    print "a() => ", a(), "\n";
};
# prints "a() => changed"  

##</code><code>##

sub localize_and_call_fn
{
    my ($locals, $fn, @args) = @_;
    local *$_ = sub { "changed" } for @$locals;
    $fn->(@args);
}

##</code><code>##

localize_and_call_fn( [qw(a b c)], \&print_a );
# prints "unchanged"

##</code><code>##

#     for (@$locals) {
#         local *$_ = sub { "changed" };
#     }

##</code><code>##

sub localize_and_call_fn_2
{
    my ($locals, $fn, @args) = @_;

    for my $sym (@$locals) {
        my $f = $fn;
        $fn = sub {
            local *$sym = sub { "changed" };
            $f->(@_);
        }
    }

    $fn->(@args);
}

localize_and_call_fn_2( [qw(a b c)], \&print_a );
# prints "changed"

##</code><code>##

# (sub {
#     local *c = sub { "changed" };
#     (sub {
#         local *b = sub { "changed" };
#         (sub {
#             local *a = sub { "changed" };
#             (\&print_a)->(@_);
#         })->(@_)
#     })->(@_)
# })->();