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use v5.12;
use warnings;
...
$str =~ s#(\d+:\d+:)(\d);(?: \1((??{$^N+1}));)+#$1$2-$3#g;
say $str;

C:/Strawberry/perl/bin\perl.exe -w d:/tmp/pm/sequencer.pl 
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

Compilation finished at Wed May 26 15:46:31