Given a bit pattern with n-bits set (eg. 0x7 == 0b111000), _next() produces the next number that contains that same number of bits sets. (eg. 0b110100 ). And when you pass that value in it produces the next (eg. 0b101100 ); and so on.

[0] Perl> $n = 0x7; print unpack 'b6', pack 'V', $n while ( $n = _next
+( $n ) ) < ( 0x7 << 3 );;
110100
101100
011100
110010
101010
011010
100110
010110
001110
110001
101001
011001
100101
010101
001101
100011
010011
001011
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Thus, replacing 1s with successive characters from the input charset, and 0s with '-', results in the required output.