package Tie::Constrained::URI;
use vars qw/@ISA/;
use Regexp::Common 'URI';
use Tie::Constrained;

@ISA = qw/Tie::Constrained/;

sub validate { $_[0] =~ /^$RE{URI}$/ }

1;

__DATA__
Usage:
    use Tie::Constrained::URI;
    my $href_ctl = tie my $href, 'Tie::Constrained::URI';
    tie my $ftpref, Tie::Constrained::URI => sub {
        $_[0] =~ /^$RE{URI}{FTP}$/
    };
 
$href_ctl can be used to change the test on the fly.

    $href_ctl->{'test'} = sub { $_[0] =~ /^$RE{URI}{HTTP}$/ }
        if $href =~ /^$RE{URI}{HTTP}$/;

##</code><code>##

tie my $obj, Tie::Constrained => sub {
    !$_[0] or $_[0]->isa('My::Frobnicator');
};