In a first time you can simplify the obfuscated code like this :

s//Ktsf/; # now $_ = 'Ktsf'
s/./chr ord($&)-5/e; # $_ = 'Fona'

$\=$/;
# By default $/ is a newline
# A newline is printed after the last print

print
# print $_ and a newline => "Fona\n"
[download]

Second time :

use strict;
print map { chr ord($_) - 5 }  ('K', 't', 's', 'f');
print "\n";
[download]

Finaly :
'K' ASCII code = 75
'F' ASCII code = 70
chr 70 eq 'F';)

HTH,
PooLPi