Here's pm_2d_comb_long.pl - a longer, annotated version of pm_2d_comb.pl:

#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

# Original code
#say for glob '{' . join('}-{' => map { join ',' => split } <DATA>) . 
+'}';

# Read raw data
# @data_lines will be: ('1 2', '3 4 5', '6 7', '8 9')
my @data_lines = <DATA>;

# Convert spaces to commas by splitting each string on whitespace
#   to form a list whose elements are joined with commas
# @spaces_to_commas will be: ('1,2', '3,4,5', '6,7', '8,9')
my @spaces_to_commas = map { join ',' => split } @data_lines;

# Join those strings with '}-{'
# $brace_dash_brace_string will be '1,2}-{3,4,5}-{6,7}-{8,9'
my $brace_dash_brace_string = join('}-{' => @spaces_to_commas);

# Add opening and closing braces
# $glob_string will be: '{1,2}-{3,4,5}-{6,7}-{8,9}'
my $glob_string = '{' . $brace_dash_brace_string . '}';

# Finally glob '{1,2}-{3,4,5}-{6,7}-{8,9}'
say for glob $glob_string;

__DATA__
1 2
3 4 5
6 7
8 9
[download]

Outputs:

$ pm_2d_comb_long.pl
1-3-6-8
1-3-6-9
1-3-7-8
1-3-7-9
1-4-6-8
1-4-6-9
1-4-7-8
1-4-7-9
1-5-6-8
1-5-6-9
1-5-7-8
1-5-7-9
2-3-6-8
2-3-6-9
2-3-7-8
2-3-7-9
2-4-6-8
2-4-6-9
2-4-7-8
2-4-7-9
2-5-6-8
2-5-6-9
2-5-7-8
2-5-7-9
[download]