See Set::IntSpan

From the doc : << Sets are stored internally in a run-length coded form. This provides for both compact storage and efficient computation. In particular, set operations can be performed directly on the encoded representation. >>

#!/usr/bin/perl -w
use strict;
use Data::Dumper;
use Set::IntSpan;

my $good_run_list = '1-5';
my $bad_run_list  = '1,3';
my ( $good_set, $bad_set, $x_set );

if ( valid Set::IntSpan $good_run_list and valid Set::IntSpan $bad_run
+_list ) { 
    $good_set = new Set::IntSpan $good_run_list;
    $bad_set  = new Set::IntSpan $bad_run_list;
}
else { print "Please verify your run lists\n" }

$x_set = $good_set ^ $bad_set;

# Or $x_set = $good_set->X($bad_set);

finite $x_set and my @elements = elements $x_set;
print Dumper \@elements;

__END__

Output:
$VAR1 = [
          2,
          4,
          5
        ];
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hth,
PooLpi