use strict;
use warnings;

my @testStrings = qw{
   abc.1.0.1.0
   bnnn.1.0.1.1
   this.that.1.0.12.0
   dots.a.plenty.here.1.0.2.1
   };

foreach my $testString ( @testStrings )
{
    my @parts = split m{\.}, (scalar reverse $testString), 5;
    my $rest = scalar reverse $parts[4];
    my $digits = join q{.}, 
        map{ scalar reverse $_ } 
        reverse @parts[0..3];
    print qq{$rest -- $digits\n};
}