Perhaps a HoHoH would do it.

#!/usr/bin/perl

use warnings;
use strict;

my %conf_table = (
  1 => {
    conference1 => {
      type => q{type1},
      comment => q{comment1},
    },
    conference2 => {
      type => q{type2},
      comment => q{comment2},
    },
  },
  2 => {
    conference3 => {
      type => q{type1},
      comment => q{comment3},
    },
    conference4 => {
      type => q{type2},
      comment => q{comment4},
    },
  },
);

for my $time (keys %conf_table){
  print qq{time: $time\n};
  for my $cnf (keys %{$conf_table{$time}}){
    print qq{\tconference: $cnf\n};
    print qq{\t\ttype: $conf_table{$time}{$cnf}{type}\n};
    print qq{\t\tcomment: $conf_table{$time}{$cnf}{comment}\n};
  }
}
[download]

time: 1
  conference: conference1
    type: type1
    comment: comment1
  conference: conference2
    type: type2
    comment: comment2
time: 2
  conference: conference4
    type: type2
    comment: comment4
  conference: conference3
    type: type1
    comment: comment3
[download]