sed/awk/perl: How to replace all spaces each with an underscore that occur before a specific string ?

[bolega]

sed/awk/perl:

How to replace all spaces each with an underscore that occur before a
specific string ?

I really prefer a sed one liner.

Example
Input :  This is my book. It is too  thick to read. The author gets
little royalty but the publisher makes a lot.
Output: This_is_my_book._It_is_too__thick_to read. The author gets
little royalty but the publisher makes a lot.

We replaced all the spaces with underscores before the first occurence
of the string "to ".

Thanks
Gnuist