use strict;
use warnings;

use 5.010;

my $deleteFromPhrase = q{i want to start deleting here};

while ( <DATA> )
{
    say substr $_, 0, index $_, $deleteFromPhrase;
}

__END__
keep keep keepi want to start deleting here blah blah
no change in this para
but change this onei want to start deleting here burble
and also change this parai want to start deleting here blarf

##</code><code>##

keep keep keep
no change in this para
but change this one
and also change this para