I agree "pure" regex isn't the way to go, but...

Win8 Strawberry 5.30.3.1 (64)  Wed 05/26/2021  9:07:19
C:\@Work\Perl\monks
>perl

use 5.018;  # need lexicals in regexes, regex extensions

use strict;
use warnings;

my @Test = (
  '43:1:1; 43:1:2; 43:1:3; 43:1:4; 43:1:5; 43:1:6; 27:3:7; 27:3:8; 27:
+3:9; 65:1:4; 65:1:18',
  '987:23:45; 987:23:46; 65:1:17; 65:1:19',
  );

for my $data (@Test) {

  print "'$data' \n";

  my $rx_base = qr{ (?> \d+ : \d+ :) }xms;
  my $rx_tail = qr{ (?> \d+) }xms;
  my $rx_sep  = qr{ (?> ;? \s*) }xms;

  my @run;

  $data =~ s{
      ($rx_base)     ($rx_tail) (?{ push @run, $^N })
      (?: $rx_sep \1 ($rx_tail) (?{ push @run, $^N })
          (?(?{ $run[-1] - $run[-2] != 1 }) (*F))
          )+
      }
      {$1$2-$3}xmsg;

  print "'$data' \n\n";

  }

^Z
'43:1:1; 43:1:2; 43:1:3; 43:1:4; 43:1:5; 43:1:6; 27:3:7; 27:3:8; 27:3:
+9; 65:1:4; 65:1:18'
'43:1:1-6; 27:3:7-9; 65:1:4; 65:1:18'

'987:23:45; 987:23:46; 65:1:17; 65:1:19'
'987:23:45-46; 65:1:17; 65:1:19'
[download]

Update: Here's another version that I think is a bit nicer. It avoids "absolute" capture group variables and backreferences. It is also not push-y, using plain scalars that are self-initializing.

Win8 Strawberry 5.30.3.1 (64)  Tue 06/01/2021 11:31:49
C:\@Work\Perl\monks
>perl

use 5.018;  # need lexicals in regexes, regex extensions

use strict;
use warnings;

my @Test = (
  '43:1:1; 43:1:2; 43:1:3; 43:1:4; 43:1:5; 43:1:6; 27:3:7; 27:3:8; 27:
+3:9; 65:1:4; 65:1:18',
  '987:23:45; 987:23:46; 65:1:17; 65:1:19',
  );

for my $data (@Test) {

  print "'$data' \n";

  my $rx_base = qr{ (?> \d+ : \d+ :) }xms;
  my $rx_tail = qr{ (?> \d+)         }xms;
  my $rx_sep  = qr{ (?> \s* ; \s*)   }xms;

  my ($start, $prev, $end);

  $data =~ s{
      ($rx_base) \K    ($rx_tail) (?{ $start = $end = $^N })
      (?: $rx_sep \g-2 ($rx_tail) (?{ ($prev, $end) = ($end, $^N) })
          (?(?{ $end - $prev != 1 }) (*F))
          )+
      }
      {$start-$end}xmsg;

  print "'$data' \n\n";

  }

^Z
'43:1:1; 43:1:2; 43:1:3; 43:1:4; 43:1:5; 43:1:6; 27:3:7; 27:3:8; 27:3:
+9; 65:1:4; 65:1:18'
'43:1:1-6; 27:3:7-9; 65:1:4; 65:1:18'

'987:23:45; 987:23:46; 65:1:17; 65:1:19'
'987:23:45-46; 65:1:17; 65:1:19'
[download]