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    #!/usr/bin/perl -l

...
        }
        return \@product;
    }

    use Data::Dumper;
    $Data::Dumper::Terse = 1;
...

    print Dumper( combinations( [1..3], ["a","b"] ) ), "\n";
    # [[1,'a'],[1,'b'],[2,'a'],[2,'b'],[3,'a'],[3,'b']]

    sub charset_combinations {
        my @charsets = map [split//], @_;
        map join("", @$_), @{ combinations( @charsets ) };
    }

my @abcees3 = charset_combinations( ("abc") x 3 );
print "@abcees3\n";
# aaa aab aac aba abb abc aca acb acc\
# baa bab bac bba bbb bbc bca bcb bcc\
# caa cab cac cba cbb cbc cca ccb ccc

    my @charsets = qw( abc 123 !@$ );

...
    # 3: a1! a1@ a1$ a2! a2@ a2$ a3! a3@ a3$\
    #    b1! b1@ b1$ b2! b2@ b2$ b3! b3@ b3$\
    #    c1! c1@ c1$ c2! c2@ c2$ c3! c3@ c3$