I'm not sure I entirely understand your requirements, but /t(?!est\b)\w*t/ would match any 't' not followed by 'est' and a word break then match 0 or more word characters and then match one last 't'. For example (anchors added):

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

/^t(?!est\b)\w*t$/ and print while <DATA>;

__DATA__
test
testset
tot
tesset
tt
[download]

Adapting it slightly for your original problem like this

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

$_ = "thought test tot 1 2 3 tesset";

s/t(?!est\b)\w*t\s*//g;

print;
[download]

-sauoq
"My two cents aren't worth a dime.";