Your substr didn't work because the code doesn't check to see if rindex found the '.' it was looking for. If you do that, you don't need a regexp at all.

if ((my $last_dot = rindex($string, '.')) > -1) {
    $result = substr( $string, 0, $last_dot + 1 );
}
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... just in case you wanted to know why your code wasn't working as expected.