#!/usr/bin/perl -w
use strict;

my $line;

for(chomp($line = <DATA>); $line =~ /(\d+)\s+([_a-z.]+)/; chomp($line = <DATA>)) {
   my $plaintext = decipher($1, $2);
   print "$plaintext\n";
}

sub decipher {
   # place code here
}

__DATA__
5 cs.
101 thqqxw.lui.qswer
3 b_ylxmhzjsys.virpbkr
0