use strict; 
use warnings;

while (<DATA>) {
    s/(i want to start deleting here).*//;
    print;
}

__DATA__
keep keep keep keep keep i want to start deleting here blah blah blah blah blah blah blah up to the newline.

keep keep keep keep keep i want to start deleting here blah blah blah blah blah blah blah up to the newline.

keep keep keep keep keep i want to start deleting here blah blah blah blah blah blah blah up to the newline.

##</code><code>##

keep keep keep keep keep 

keep keep keep keep keep 

keep keep keep keep keep