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.

In reply to Re: Trying to delete text from a text file by toolic
in thread Trying to delete text from a text file by nervousmark

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