bannor> cat 1056884.pl 
#!/usr/bin/env perl
use Modern::Perl;

open my $wf, '<', '2of12.txt' or die $!;
my @words;
while (<$wf>) {
    chomp;
    push @words, lc($_) if length($_) > 1 and length($_) < 9 and ! /-/;
}
@words = keys { map { $_ => 1 } @words };

say "Working with @{[scalar @words]} words";

my @z;
for my $w (@words) {
    my @letters = split //, $w;
    for my $e (@z) {
        my $quick = join '', sort split //, $w . $e->[0];
        $quick =~ s/(.)\1+/$1/g;
        next if length($quick) > 8; # bail out early if no chance
        my $fill = $e->[0];
        my $save = $fill;
        my $new = '';
        for my $l (@letters) {
            unless( $fill =~ s/$l// ){
                $new .= $l;
            }
        }
        $save .= $new;
        if ( length($save) < 9 ) {
            push @$e, $w;
            $e->[0] = $save;
        }
    }	
    push @z, [$w, $w];  # start a new stack with this word
}

my $e = ( sort { @$b <=> @$a } @z )[0];
say "Letters: ", shift @$e;
say "Number of words: ", scalar @$e;
say "  $_" for sort @$e;

bannor> time perl 1056884.pl
Working with 21664 words
Letters: soartple
Number of words: 198
perl 1056884.pl  3628.31s user 0.14s system 99% cpu 1:00:34.69 total

## Words found:
ale, alert, aloe, alp, also, alter, alto, ape, apostle, apse, apt, are,
arose, art, arts, as, asp, aster, ate, atop, ear, earl, east, eat, eats, era,
eta, la, lap, lapse, laser, last, late, later, lea, leap, leapt, lest, let,
lo, lop, lope, lore, lose, loser, lost, lot, lots, oar, oat, ole, opal,
opera, opt, or, oral, orate, ore, pa, pal, pale, par, pare, parole, parse,
past, paste, pastel, pastor, pat, patrol, pea, peal, pear, peat, pelt, per,
pert, peso, pest, pet, petal, petrol, plat, plate, plea, pleat, plot, pol,
polar, pole, polestar, pore, port, pose, poser, post, postal, poster, pot,
prate, presto, pro, pros, prose, rap, rape, rasp, rate, re, real, reap, rep,
repast, rest, roast, roe, role, rope, ropes, rose, rot, rote, sale, salt,
sap, sat, sate, sea, seal, sear, seat, sepal, septa, sera, slap, slat, slept,
sloe, slop, slope, slot, so, soap, soar, sol, solar, sole, sop, sore, sort,
sorta, sot, spa, spar, spare, spat, spate, spear, spelt, splat, spore, sport,
spot, sprat, stale, staple, stapler, star, stare, step, stop, store, strap,
strep, strop, tale, tape, taper, taps, tar, tare, taro, tarp, tea, teal,
tear, to, toe, tole, top, tops, tor, tore, trap, traps, trope, tsar