A minor nit in terminology: Your program doesn't modify the file in place. "Modify in place" means to open the file in read/write mode and alter the file without creating a new copy. (Not terribly safe for many applications, though!) Here's an example of modifying a file in place:

$ cat 659032.pl
#!/usr/bin/perl -w
use warnings;
use strict;

# Open in Read/Write mode and modify it in place
open FH, "+<", "a_file.txt" or die "Can't open file!";
print FH "Foo bar baz!";
close FH;

$ cat a_file.txt
Now is the time for all good men to
come to the aid of their party.

$ perl 659032.pl

$ cat a_file.txt
Foo bar baz!ime for all good men to
come to the aid of their party.

$
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