The following should get you started. It uses Perl's substitution operator to delete everything to the end of the line:

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.
[download]

prints out:

keep keep keep keep keep 

keep keep keep keep keep 

keep keep keep keep keep
[download]

Since you are new to Perl, I will recommend reading through some of the online tutorials from Perl's official documentation website, starting with perlintro.