I'd never thought it through particularly, since in my case it is much less likely anyone else will have to read my code, but readability does seem to be quite a personal thing. I think there are some styles that are going to be less readable for almost everyone, but a lot of cases where it's totally dependent on the reader. I probably betray my own sloppy habits when I say I found your version one easier to understand than version two.

My {pre-decimalisation pound} / 120 is that the map version wd be more readable like this:

my @files = map {&makefiles} @in;

sub
makefiles
{
    my ($count, $file) = split;

    $file =~ s/^$in_prefix//;

    my ($campaign, $month, $fname) = split /\//, $file;

    return {        # not strictly necessary, but *readable*
        campaign => $campaign,
        month => $month,
        file => $fname,
        count => $count,
        sort => "$campaign:$month:$file"
    };
}
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§ George Sherston