What kind of size of file are you talking about? (i.e. is it acceptable to slurp into memory)

one way is to just read a line at a time, and skip a chunk after your marker .. then write the whole thing back out..

my @lines;
open FILE, "<", "foo.txt";
while(  ){
  push @lines, $_;
  next unless /\byourWord\b/;
   for 1 .. 5;
}
close FILE;
open FILE, ">", "foo.txt";
print FILE, @lines;
close FILE;

Hmm.. that removes the 5 lines after each match .. re-reading OP it seems like it mightbe asking for just delete all the lines after the match ... using similar method:

my @lines;
open FILE, "<", "foo.txt";
while(  ){
  push @lines, $_;
  last if /\byourWord\b/;
}
close FILE;
open FILE, ">", "foo.txt";
print FILE, @lines;
close FILE;

Another option is [cpan://Tie::File] and a similar approach to above .. or start looping through the array, and just splice off everything after your word is found.

This sounds like a good job for [cpan://Tie::File]. (the following is untested.)

use strict;
use warnings;

use Tie::File;

my @array;
tie @array, 'Tie::File', 'filename.txt' or die $!;

foreach ( 0 .. $#array ) {
    if( $array[$_] =~ /word/ ) {
        $#array = ( $_ < $#array ) ? $_ + 1 : $_;
        last;
    }
}

untie @array;

I often think of [cpan://Tie::Array] as mostly a novelty; not really something you would think of actually using. But I shouldn't think of it that way; it works, it's fast, and it makes some simple tasks even simpler.

If the files aren't huge, or the word appears fairly early (a few hundred MB) in the file, then you can do this with a "one-liner" (wrapped for viewing; adjust quoting to your OS needs):

perl -e"BEGIN{$/=qq[the word or phrase]}" 
-ne"$n=tell ARGV; close ARGV; truncate $ARGV, $n" path\to\the\file

You can, but these simpler tasks are easier to do with an editor unless the file is very large:

ed file <<<$'/\/+,$d\nw'

Alternately replace ed with ex, the same command will work with it.