I like the potential flexibility there, although I'll also (for the sake of other readers and devil's advocacy) point out that if a caller wanted to put off overhead of loading a module, it's best done in a require() call in whatever conditional requires that module.

Actually, all you're putting off is running the import (which in an OO class you aren't likely to be running anyway). All of the code sitting at the class level will still run:

package Foo;
use strict;
use warnings;

BEGIN {
    print "BEGIN called\n";
}

print "Class level logic\n";

sub new {
    my $class = shift;
    return bless {}, $class;
}

1;
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In the above both print statements run, regardless of whether you use or require the package.

Not to say efficiency is a bad thing, but I also try to avoid premature optimization

Hmm, maybe I shouldn't have mentioned the additional side-benefits. Its not an attempt at optimisation, its merely a side-benefit of the design. At the end of the day though, its really just a matter of style. If you find it easier to put your initialisation logic at the class level, then thats where it should go. More important than how things should be done is consistency in your code-base :)