The cookbook version is better, but I'm not sure I'd like to load File::Basename just for this. I'd probably end up writing it as:

sub extension {
    my $path = shift;
    my $ext = (split /\./, $path)[-1];
    return $ext;
}
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Update: and since we all know this is going to devolve into Golf sooner or later, here's a shortened version of that:

sub extension {
    (split/\./,$_[0])[-1];
}
[download]