use strict;
use warnings;

my $input  = 'ABC---';

sub insert {
  my( $sofar, $chars, $dashes ) = @_;
  print "$sofar\n" and return unless @$chars or @$dashes;
  insert( $sofar . $$chars[0],  [ @$chars[ 1..@$chars-1] ],   [ @$dashes ] ) if @$chars;
  insert( $sofar . $$dashes[0], [ @$chars ], [ @$dashes[ 1..@$dashes-1 ] ] ) if @$dashes;
}

insert( '', [ $input =~ /[^-]/g ], [ $input =~ /-/g ] );