Why even use cut, awk, or sed to trim when you could just use the shell parameter replacement "${i%TMB}JPG". But to keep it perl:

while (<*.TMB>) {
 my $new = $_;
 substr($new, -3) = "JPG";
 warn "$new already exists", next if -e $new;
 rename $_, $new or warn "Error renaming $_: $!";
}
[download]

Update: I just saw only the above node when I answered, I now see that except for my first comment on the shell solution, I've posted practically the same code already posted higher up (great minds think alike I suppose) :0)