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

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

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