$ echo -en "hello\nworld\n" >foo.txt
$ perl -i -ple '$_ = ucfirst' foo.txt
$ cat foo.txt 
Hello
World

$ echo -en "quz\nbaz\n" >bar.txt
$ perl -MFile::Replace=-i -ple '$_ = ucfirst' bar.txt
$ cat bar.txt
Quz
Baz