$ 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.

$