I admit I've read easier stuff, but I (think I) got there after not too long ...

IMO, the OP has, as its basis, an array of integers (each in the range 0 .. 31) - where sequences of consecutive numbers e.g. 2,3,4,5,6 can be shortened using a colon ':' e.g. 2:6 (but not necessarily in ascending order). The problem laid out in the OP can be summarised as the solving of the following 2 problems...

1. expand the/any ranges (colon connected digit sequences) in the list.
2. determine (and lists), numbers in the same range i.e. 0 .. 31, that are missing from i.e. don't appear in, the array (containing range expansions).

Update:

use warnings;
use strict;

my @data = qw/0 6:4 12:8 19:31/;
my %nums = map { ( $_ => undef ) } 0 .. 31;
map {
    local @_ = sort { $a <=> $b } split /:/;
    for (my $n = $_[0]; $n < ( @_ == 1 ? $_[0] + 1 : $_[1] + 1 ); $n++
+ ) {
        delete $nums{$n};
    }
} @data;
print "$_\n" foreach sort keys %nums;
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Yes, I figured out what he meant once he reworded the OP.

And you didn't even know bears could type.