One thing that immediately jumped out at me is the question of why you're iterating over the keys of the hash just to determine if each of the individual hash keys match the word you're comparing from your input file. One of the express reasons for using a hash is so that you don't have to iterate over each element to find one.

If my task description were, "Read a text file, compare it to a banned word list, and then rewrite the text file, minus the banned words." I would probably do it something like this. ...note, this isn't a cut-and-paste solution for you, it's just an example from which you hopefully can derive a solution taylored to your needs...

use strict;    # Please do this!
use warnings;  # And this!

# First, read from the <DATA> filehandle to grab the list
# of banned words.  map the banned words into hash-keys.
my %banned = map { chomp $_; $_=> undef } split /\s+/, <DATA>;
my $infile = "textfile.txt";
my $tmpfile = "outfile.tmp";

open my $in, "<$infile" or die "Cannot open $infile.\n$!";
open my $out, ">$tmpfile" or die "Cannot open temp output file.\n$!";

while ( my $inline = <$in> ) {
    chomp $inline;
    my $printline = "";
    foreach my $word ( split /\s+/, $inline ) {
        next if exists $banned{ lc $word };
        $printline .= $word;
    }
    print $out "$printline\n";
}
    
close $out;
close $in;
rename $tmpfile, $infile;

__DATA__
a at be for and to of in the as i it are is am on an you me b c d
e f g h j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 10
[download]

I hope this helps... good luck.