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sub mergesort {
    my( $av, $beg, $end )= @_;
...
        }
    }
}

sub uniqmergesort {
    my( $av, $beg, $end )= @_;
...
    return 0;
    #^^^^^^^^
}

sub merge {
    my( $av, $a, $b, $c, $d )= @_;
...
    @$av[$beg..$end]= @$av[ @i, $a..$b, $c..$d ];
    return $d-$end;
}

(...done...)= items already sorted and (less dups) output
p= the pivot or a duplicate of it
...
if n is a  "b", swap:  b<------->u
if n is an "a", swap:            p-a
if n is a  "p", swap:            p a<------->p

(...done...) b b b b b b b b p a a a a a a a p p p p (...to-do...)
            |---sort next---|

(...done...) b p a p p (...to-do...)
            |write|