archer:

You could try something like:

sub is_interesting {
    my $name = shift;

    # only directories are interesting
    return 0 unless -d $name;

    # and only those containing input.xml and output.xml
    return 0 unless -f $name . "/input.xml";
    return 0 unless -f $name . "/output.xml";

    return 1;
}

sub preproc {
    return grep { is_interesting($_) } @_;
}
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...roboticus

When your only tool is a hammer, all problems look like your thumb.