use strict;
use warnings;

print join "\n", comb('A'..'C');

BEGIN {
    my @c_out;
    sub comb {
        @c_out = ();
        permute('', $_, @_) for (0..$#_);
        return @c_out;
    }
    sub permute {
        my ($str, $depth, @chars) = @_;
        if (!$depth--) {
            push @c_out, $str.$_ for @chars;
        }
        else {
            permute($str.$chars[$_], $depth, @chars[($_+1)..($#chars)]
+)
                for (0..$#chars);
        }
    }
}
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