I'm with merlyn on this. Except in the odd case of someone writing

    package A;
    use base qw(B);
    @ISA=(C);
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there's no opportunity for @ISA to have been initialized prior to it being set explicitly. You're trying to warn people off from a demon that isn't there. Or at least I've never seen it.

I do agree, though, that use base is a better way to go.