Specifically, consider using the Safe module to restrict the kinds of things that the external code is allowed to do. For example, the following:

use Safe;
my $compartment = new Safe;
$compartment->permit_only(':base_core');
my $result = $compartment->reval($foo);
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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.

Makeshifts last the longest.