use v5.12;
use warnings;

my $str ='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';
say $str;

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