The docs for substr are mostly straightforward, but it won't really do precisely what you're looking for:

$raw = ".declare book_num                     a13";
$var = substr $raw, 9;
print $var;

# prints "book_num                     a13"
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However, you could also try split. I don't think there would be a noticeable performance difference either way, but splitting would also give you the value if you need it:

(undef, $var, $val) = split /\s+/, $raw, 3;
printf '%s=%s', $var, $val;

# prints book_num=a13
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