#$file contents below....
#Line1 - Blah
#Line2 - Blah
#Line3 - Blah
#Line4 - Blah
#Line5 - Blah

tie @file_lines, 'Tie::File', $file or die;
$file_lines[3] = "This is my new record on line 3,\n";
untie @file_lines or die "$!";

#$file contents now has (on no format text editors)...
#Line1 - Blah
#Line2 - Blah
#Line3 - This is my new record on line3,Blah
#Line4 - Blah
#Line5 -