AFAIK, you can't. That's the whole point about local. Any good reason why you need to use local instead of my? You may try giving a peek into Coping with Scoping and Seven Useful Uses of local for insightful info.

As a side note, read perldoc perlvar to find out why as a general rule $a should not be used as a general purpose variable.

Update: all in all your test code may have been written like thus:

#!/usr/bin/perl -l

$c='main';

{
        local $c='sub';
        print $c;
        print $main::c;
}

print $c;

__END__
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