package Tie::Substr;
use base 'Exporter';
@EXPORT = 'substr';

sub TIESCALAR { bless [ $_[1], 0 ], $_[0]; }

sub FETCH {
    $_[0][1] = 1;
    @{ $_[0][0] } == 2 and return
        substr($_[0][0][0], $_[0][0][1]);
    @{ $_[0][0] } == 3 and return
        substr($_[0][0][0], $_[0][0][1], $_[0][0][2]);
    @{ $_[0][0] } == 4 and return
        substr($_[0][0][0], $_[0][0][1], $_[0][0][2], $_[0][0][3]);
    die;
}

sub STORE {
    $_[0][1] = 1;
    @{ $_[0][0] } == 2 and return
        substr($_[0][0][0], $_[0][0][1]) = $_[1];
    @{ $_[0][0] } == 3 and return
        substr($_[0][0][0], $_[0][0][1], $_[0][0][2]) = $_[1];
    eval 'substr($foo, 0, 0, 0) = ""'; die;
}

sub DESTROY {
    $_[0]->FETCH unless $_[0][1];
}

sub substr : lvalue {
    tie my $foo, 'Tie::Substr', \@_;
    $foo
}