Well, these are just my suggestions; all of which you can take or leave as you see fit. Most of them are just opinion. Remember, TIMTOWTDI! =)

1. for (my @files=(glob "$ENV{HOME}/$ENV{SANDBOX}/examples/*")){
   In this instance, the array @files, as well as the variable $index are synthetic variables. An easier, perhaps more elegant, way of doing this would be to use an array slice. Update: Unfortunately, glob() doesn't actually return an array. So I can't figure out a way to not use a temporary, synthetic variable (@files) to do the slice. Someone please let me know if I'm missing something.
   
   Since the for loop stores the current item in $_, there is no need for the $files[$index] construct inside the loop. If you'd like a named variable index, you can always use the "for my $index (...)" form.
2. print while <SCRIPT>;
   Hrm. Well, the main thing with this that I can see is: What if the snippet is longer than one screen in length? You might consider using an external program to list the file (less?) or to at least use a counter to pause after a screenfull of info. Also, you might consider allowing an external editor (vi || emacs), as it would have syntax highliting.
3. do $files[$index]; You want to execute these snippets? What if they don't work so well as stand-alones (as snippets are wont to do)? Oh well. That's your choice.
4. redo STUDY unless <> =~ /^[Nn]/;
   Um... How do you exit the program? I guess you have to view all the files, eh? Maybe another option would be in order here. Also, STUDY isn't really needed (though it doesn't really hurt either).

Update: Well, that'll teach me to post in the middle of the night w/o testing my code first. =) I forgot to "my" (or "use vars") $EDITOR and $PATH. For some reason, I failed to realize that using an external viewer obviates the need to open the file at all. I forgot a comma on the print line. And my array slice totally didn't work (see above). Oh, and I also didn't notice (and thus perpetuated) that mkmcconn was using 1 as the default starting index (which skips the first file). *sigh* No more posting from home for me, that's for sure!

At any rate, this is how I would rewrite it:

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

my $EDITOR = "/usr/bin/less";
my $PATH = "$ENV{HOME}/$ENV{SANDBOX}/examples";

my @files = glob("$PATH/*");
for (@files[($ARGV[0] || 0) .. $#files]) {
    system($EDITOR, $_);
    # do $_;  # You can do that if you want to...  Pun not intended.

    # Personally, I'd make Next the default... But whatever. =)
    print "Type [N]EXT to see the next snippet, [Q]UIT to quit,\n",
          " or any other key to repeat the last snippet: ";

    my $input = <>;
    $input =~ /^N/i && next;  # or: next if($input =~ /^N/i);
    $input =~ /^Q/i && last;  # or: last if($input =~ /^Q/i);
}
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Well, that's my $0.02. Let me know what you think!