It's not working because you're only replacing the first part, not the entire thing. Try this:

$var = ".declare book_num                     a13";
$var =~ s/^\.\w+\s+(\w+).*$/$1/g;
[download]

Or, forget the substitution:

$var = ".declare book_num                     a13";
$var =~ /^\.\w+\s+(\w+)/;
print $1;
[download]

(Make sure to check that there /was/ a match, of course). If you haven't already, check out the relevant perldocs as well, such as perlretut.

Update: Whoops, ForgotPasswordAgain beat me to it.