If you define a sub within a sub it unfortunately gets imported into the the current package's symbol table. A possible solution would be

use strict;

sub x {
    print "x() in main::\n";
}

sub a {
    local *x = sub { print "x() in a()\n"; };
    x();
}
sub b {
    local *x = sub { print "x() in b()\n"; };
    x();
}
&a();
&b();
&x();
[download]

But this assumes that *x is declared within the same package as a() and b().
HTH

broquaint

update [2002-05-20]: you could also check out the beginnings of a small module that attempts to give the illusion of having lexically scoped subs.