Here's one possible solution. No attempt was made to match the order in the generated sequence.

use Algorithm::Combinatorics qw( permutations );

my $iter = permutations( [split //, shift // q(how do you do)] );

while (my $p = $iter->next) {
    print join q(), @$p, "\n";
}
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See Algorithm::Combinatorics for more info and examples.