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

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

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