#!/usr/bin/perl -w
use strict;
use warnings;

my $str = "this.is.example.text";
print "Before: $str\n";
$str =~ s/^(\w+\.\w+\.)(\w+\.\w+)$/[$1] [$2]/;
print "After : $str\n";

__END__
Before: this.is.example.text
After : [this.is.] [example.text]