Yes, it seems that you've figured it out by now, thanks to AnamolousMonk: the easiest way for achieving what you want is a negative look-ahead assertion:

my $regex = qr/\w{3}_[tr]x_[pn]\d(?!\d)/;
[download]

But even if you did not know about zero-width assertions, you could still have obtained more or less the same resukt with an alternation:

my $regex = qr/\w{3}_[tr]x_[pn]\d(\D|$)/;
[download]

which means: match everything to the required digit, followed by either a non-digit or the end of the string.

TIMTOWTDI.