Update: 108 passing strict and warnings :-)
use strict;

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

sub decipher {
$,=0;($;,$_)=@_;@;{@;{0..27}=('_','a'..'z','.')}=0..27;$;[$;*$,++%@_]=
+$;{($;{$_}+$,)%28}for@_=/./g;join'',@;
}

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

[download]

Here is my initial trial at 126 chars. It passes strict and warnings.

sub decipher {
my($k,@r,%b,@s)=@_;@s=pop=~/./g;@b{@b{0..27}=('_','a'..'z','.')}=0..27
+;$r[$k*$_%@s]=$b{($b{$s[$_]}+$_)%28}for 0..$#s;join'',@r
}

[download]
Aziz,,,

In reply to Re: (Golf) Untwist by abstracts
in thread (Golf) Untwist by ebbeatty

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