According the the documentation for File::Find:

The wanted function takes no arguments but rather does its work through a collection of variables.

    $File::Find::dir is the current directory name,
    $_ is the current filename within that directory
    $File::Find::name is the complete pathname to the file.

The above variables have all been localized and may be changed without affecting data outside of the wanted function.

So this line:

return unless $File::Find::dir =~ /[IPD]\d{8}$/;
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is actually testing the parent directory, not the current file. Better to run both tests against the current filename in $_:

sub dir_names
{
    return unless -d $_;
    return unless $_ =~ /[IPD]\d{8}$/;
    $dirs{$File::Find::name} = 1;
}
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or just:

sub dir_names
{
    return unless -d;
    return unless /[IPD]\d{8}$/;
    $dirs{$File::Find::name} = 1;
}
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I think that fixes the problem.

Hope that helps,