if ( -s "$path_only/$file_name" > $last_maxsize ) {
    local $/ = \$last_maxsize;
    open IN, '<', "$path_only/$file_name";
    $_ = <IN>;
    close IN;
    open OUT, '>', "$path_only/$file_name";
    print OUT;
    close OUT;
}

##</code><code>##

    while ( my $new_record = <$last_in> ) {      
        next if -s $new_record > $last_maxsize;
        ...