Regexp::Common is a very good partner for this tied technique - I had it in mind when I wrote Tie::Constrained. Here's an example of a subclass, Tie::Constrained::URI, where a tied variable is by default restricted to be a well-formed URI.

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}$/;
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As it stands, I think that Params::Validate compatibility is for the future, but it's a very good idea. I'll probably need to rename my validate class method again.

Many of the simpler things Params::Validate does for objects can be imitated with scalar Tie::Constrained as it is:

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

The first term is so that $obj can be undefined to break circular references.

Thank you much for the suggestions, I'll take them very seriously.