Sigh - use strict; use warnings;, post a complete sample that demonstrates the problem. As usual, here's a starting point:

use strict;
use warnings;

while (my $line = <DATA>) {
...
}

__DATA__
...
[download]

Dodgy stuff in your first post:

$line=; # Just plain wrong!

while ($line=~m/([123]?[.s]*[a-z\=.]+)/oi) # Where does $line get a va
+lue?

$t_book=~s/\l=([a-z]+)\/$1/ig; # The \/ is wrong!

$t_book=~s/<(|/)(B|I|SC|U)>//ig; # The |/ is wrong (and why would you 
+want to capture it?)
[download]