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@pick      @arr[@pick]

...
1 2 4  ==>  . x x . x
1 3 4  ==>  . x . x x
2 3 4  ==>  . . x x x

return @$arr[ @pick = ( 0 .. $num - 1 ) ]
    unless @pick;

my $i = $#pick;
$i-- or return
    while ( $pick[$i]++ == @$arr - $num + $i );

@pick[$i .. $#pick] = $pick[$i] .. $#$arr;