use strict;
use warnings;

my $input  = 'ABC---';

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

insert( '', $1, $2 ) if $input =~ /^([^-]+)(-+)$/;