For an excellent definition of modern Perl, I'm honoured to quote the late Iain Truskett aka Spoon aka Koschei:

Check that the tutorial uses good, modern style. 3-arg open (or at least whitespace between action and filename). Should be advocating 'use strict' and 'use warnings' (which will show it's at least 5.6 or later). Should introduce perldoc very early on. Lexical filehandles. No ampersands when calling functions in the ordinary case. /x both mentioned and advocated for regex. POD. Other stuff. And most importantly, security, including common sense and tainting.

In "Other stuff" above, I'd include a sound Test::More-based test suite (in t/ directory) with good code coverage.

Also, merlyn comments on a script that is not written in modern Perl style here.

FWIW, I'd say Perl become "modern" around version 5.6, since that is the version that introduced lexical file handles, 3-argument open, and use warnings.