$str = "abc.def.ghi.jkl";
$str =~ s{
  .*     # match as much as you can
  \K     # and then pretend HERE is where we start
  \. .*  # then a . and anything else
}{}x;    # replace with nothing

__END__
abc.def.ghi.jkl       $&
AAAAAAAAAAA     .*    "abc.def.ghi"
                \K    ""
           B    \.    "."
           BCCC .*    ".jkl"