There are several reasons why this proposal wouldn't work, or would be a bad idea.

First, optional tie methods have the habit of breaking inheritance. Consider a hypothetical popular CPAN module that uses a tied SCALAR. Suppose my code creates a subclass of this, where I override FETCH, with something like

    sub FETCH {
        my $self = shift;
        do_domething;
        $self->SUPER::FETCH(@_);
    }
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Now suppose that the CPAN maintainer updates his code to include a new FETCHSUBSTR method. I install this version, and suddenly my FETCH method is mysteriously no longer being called.

Second, given the way the perl5 interpreter works, it's not practical to combine array subscripting and substr, so it wouldn't be possible to use *SUBSTR with arrays and hashes.

Lastly, it would help if you explained how the various uses of the substr function were mapped to *FETCH calls, especially when substr is used as an lvalue, eg

$ perl586 -wle 'sub f { $_[0] = "x" } $a = "abcd"; f(substr($a,0,2)); 
+print $a'
xcd
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Dave.