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#!/usr/bin/perl
use warnings;
use strict;

package Constrained;
use Errno qw/EINVAL/;

sub _invalidate {
    $! = shift;
    die sprintf('Constraint violation: %s by %s::%s in %s line %s.',
...
                map { qq($_) } (caller 1)[0,3,1,2] ),
        "\n";
}

sub TIESCALAR {
    my $class = shift;
    if (defined $_[1]) {
...
    }
    bless { code => $_[0], val => $_[1]}, $class;
}

sub STORE {
    my ($self, $val) = @_;
    $self->{code}($val) or _invalidate EINVAL;
    $self->{val} = $val;
}

sub FETCH {
    defined $_[0]{val} or _invalidate EINVAL;
    $_[0]->{val};
...
}

1;

package main;
my ($v, $c) = (qr/[aeiouy]/i, qr/[bcdfghjklmnpqrstvwxyz]/i);

...
    tie my $foo, 'Constrained', sub {shift=~/^$c$v$v$c$c$/};
    sub foo () :lvalue { $foo }
}

print q(Testing FETCH error for undefined value,), $/; 
defined( eval { print 'foo is ', foo, $/ }) or print $@, $/;

...
      }) ?
      ('baz is ', baz, $/, $/) :
      ($@, $/);

__END__
Testing FETCH error for undefined value,
Constraint violation: Invalid argument by main::Constrained::FETCH in 
+lvval.pl line 50.
...

Testing TIESCALAR error valid value, 'suits',
baz is suits