Well, you could go with

use constant PADDING => 2;

my $col_width = PADDING + (
    sort {
        $b <=> $a
    } map {
        length
    } keys %list, map {
        @$_
    } values %lists
)[ 0 ]; ## Longest key or value
[download]

But in

use constant PADDING => 2;

my $col_width = PADDING + (
    sort {
        $b <=> $a
    } map {
        length
    } map {
        ref ? @$_ : $_
    } %lists
)[ 0 ]; ## Longest key or value
[download]

The line ref eq 'ARRAY' ? @$_ : $_ just says: if it($_) is a value (array) flatten it to a list, (which you already had in your own version), or if it is a key, pass it through untouched.

That avoids having to treat the keys and values separately, (and iterate the hash twice to generate them). In doing so it reduces the noise level a little further.