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

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

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