Specifically, consider using the Safe module to restrict the kinds of things that the external code is allowed to do. For example, the following:
will forbid a great number of operations but leaves enough allowed so that $foo could be a configuration file - written in Perl syntax. In that case, the rdo() method is also interesting:
my $result = $compartment->rdo($filename);
which is a safe replacement for my $result = do $filename; For information about the tags and names you can use in the permit() call, see the documentation to the Opcode module.
my $compartment = new Safe;
my $result = $compartment->reval($foo);
Makeshifts last the longest.