That can be done easily with an object that takes charge of the reblessing and which reverts what it did at destruction time. Your illustration code would look like:

package boo;

sub to_s { "a boo" };

package bar;

sub to_s { "a bar" };

package main;

sub doit {
  my $boo = shift;

  my $reblesser = Reblesser->new;

  $reblesser->rebless($boo, "bar"); 
  print "within: ", $boo->to_s, "\n"; # "a bar"

  # here the scope of the reblesser ends and so the reblessing it did
}

my $obj = bless {}, 'boo';
print "before: ", $obj->to_s, "\n"; # "a boo"
doit($obj);
print "after: ", $obj->to_s, "\n"; # "a boo" again
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The output should be:

$ perl reblesser.pl
before: a boo
[DEBUG] reblessing boo=HASH(0x102efcf8) to bar
within: a bar
[DEBUG] reverting blessing on bar=HASH(0x102efcf8): back to boo
after: a boo
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This way you just have to be sure that the scope in which you want to limit the reblessing is the same as the scope of the reblesser object. A trivial implementation of such reblesser would be:

package Reblesser;

use strict;
use warnings;

use base qw(Class::Accessor);
__PACKAGE__->mk_accessors(qw(reblessed));

use Scalar::Util qw(blessed);

# reblessed holds pairs [ (object => original_package) ]

sub new {
    my $self = shift;
    return $self->SUPER::new({ reblessed => [] });
}

sub rebless {
    my $self = shift;
    my $obj = shift;
    my $package = shift;
    die "not blessed" unless blessed $obj;
    push @{$self->reblessed}, [ $obj, blessed $obj ];
    warn "[DEBUG] reblessing $obj to $package\n";
    return bless $obj, $package
}

sub DESTROY {
    my $self = shift;
    for (@{$self->reblessed}) {
        my ($obj, $pkg) = @$_;
        warn "[DEBUG] reverting blessing on $obj: back to $pkg\n";
        bless $obj, $pkg;
    }
}
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