The main difference between the three approaches is merely syntax :-)

Ok, first the requirement is that the various special module variables such as:  @ISA, @EXPORT_OK, $VERSION have to be package variables (not lexicals). One way to have package variables without having strict complain is simply to not turn on strict until you are done referring to such variables (that is, initializing or otherwise explicitly using the variables). Any time you use a variable (that hasn't been declared) is is automatically a package variable belonging to the current package. In general this isn't a great way to deal with global/package variables --- but, in the case of these special module variables, you usually only use them once (initialization) and that's it, so it seems acceptable in my mind to defer turning on strict until after these variables are initialized.

Another way to handle global/package variables and strict is to declare such variables with the 'use vars' pragma --- this essentially tells strict that these variables are OK to use without full package qualification. This is perhaps the best alternative of the three (see below for why).

Finally, with 5.6+ there is the new declaration our(). This does what 'use vars' does except that it is a lexically scoped declaration --- ie, you can refer to such variables without package qualification for the duration of the current lexical scope (in this case the whole file). I generally don't find our very useful, and it also means your module is restricted to being used with versions of Perl at 5.6 or higher. On the other hand, you don't have to type the variable twice (with vars you need to declare it, then you can initialize it, with our you can do both in one statement). The h2xs utility (which eases building module packages) now uses our as the method of choice.

All this talk of package variable qualification brings up a fourth alternative --- simply using fully qualified package names for the variables.

package Blah;
use strict;
require Exporter;
@Blah::ISA = qw/Exporter/;
@Blah::EXPORT_OK = qw/whatever/;
#...
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Using fully qualified names instead of declaring them with vars or our is not recommended for two reasons: 1) you lose any typo-checking that strict gives if you use the variable in more than one place, and 2) who wants to type that much? --- and 3) it looks kind of ugly, no?

Which one should I use? Well, TMTOWDI, but I would recommend using the vars approach for the following reasons: a) works with earlier versions of Perl; b) you still get strict's typo checking (assuming you spelled them correctly the first time), and c) if you need other package variables (for exporting or whatever), you declare them right along with the special module variables:

package Blah;
use strict;
use vars qw/@ISA @EXPORT_OK $VERSION $Something_else/;
#...
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I hope that clarifies rather than confuses the issue.