When ->isa checks are used with respect to the delegate for the prupose of type checking (what kind of archive format do you encapsulate, mr Archive::Any instance?), then it's perhaps a design problem, because not caring is the reason we wrapped it in the first place.
But one may find desirable to have some common functionality plus extra type-specific features, e.g. 
if ($this->isa 'cool_archive') {
    print $this->name, " is a cool archive.\n", <<EOT;
You also have the following cool features:
... (more cool features)
EOT
    # interactively ask user to pick one...  
}

[download]
Or is such a design to be considered inherently risky?

In reply to Re^4: Modification of @ISA at run time by blazar
in thread Modification of @ISA at run time by jkeenan1

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