How is this:

use strict;
use warnings;
my @a = qw(0 6:4 12:8 19:31);
my %b;
for (@a){
   if (!/:/){
      $b{$_}=1;
   } else {
      my($a,$b) = split(/:/, $_);
      ($a,$b)=($b,$a)   if $a>$b;
      $b{$_}=1  for($a..$b);
   }
}
print join(" ", grep {!exists $b{$_}} (0..31)), "\n";
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Update:

In avoidance of tax filing, I carried on... You asked for 13:18, and later 18:13. This converts 0 6:4 12:8 19:31 to 1:3 7 13:18.

use strict;
use warnings;
my @a        = qw(0 6:4 12:8 19:31);
my $s        = '';
vec($s,0,32) = 0;
for (@a){
   if (/(\d+):(\d+)/){
      vec($s,$_,1) = 1  for(($1<$2) ? $1..$2 : $2..$1);
   } else {
      vec($s,$_,1) = 1;
   }
}
my @bits     = split(//,unpack("b*",$s));
my $series   = 0;
for(0..31){
   if ($bits[$_] eq 0) {
      if (!$series) {
         print;
         $series = $_;
      }
   } else {
      if ($series) {
         print ( ($series+1 < $_) ? ':'.($_-1).' ' : ' ');
      }
      $series = 0;
   }
}
[download]