This should do the trick for you, and should be cross platform as well (you'll all let me know won't you? ;)

#!perl -w

use File::Copy;
while (defined(my $filename = <*.tmb>)) {
    my $filename_2 = substr($filename,0,length($filename)-4)."t.jpg";
    move($filename,$filename_2);
}
[download]

personally I would use "_t.jpg" as opposed to "t.jpg" simply because I find it easier to read "pic_t.jpg" over "pict.jpg", but that's just my opinion ;)

ryddler