First off, use strict warnings and diagnostics or die even for code of this size. You had a typo with @nowshow that strict would have picked up. One might also want to consider use CGI or die; if this project involves handling form input. If this is all that it is, though, you might be able to escape without using it.

All that aside, I'd probably do something like:

#!/usr/bin/perl -w
use strict;

print "Content-type: text/html\n\n";

opendir(DIR,"./") or die "Couldn't open directory";
my @files = readdir(DIR);
closedir(DIR);

my @noshow = (".", "..", __FILE__);

foreach my $f (@files) {
  print "$f<br>\n" unless grep {$f eq $_} @noshow;
}
[download]

perl -pe '"I lo*`+$^X$\"$]!$/"=~m%(.*)%s;$_=$1;y^`+*^e v^#$&V"+@( NO CARRIER'