This is my reading of the spec:

#!/usr/bin/perl

use strict;
use warnings;

use Regexp::Common;

my $dur = qr /-?                           # Optional leading minus.
              P                            # Required.
              (?=[T\d])                    # Duration cannot be empty.
           (?:(?!-) $RE{num}{int} Y)?      # Non-negative integer, Y (
+optional)
           (?:(?!-) $RE{num}{int} M)?      # Non-negative integer, M (
+optional)
           (?:(?!-) $RE{num}{int} D)?      # Non-negative integer, D (
+optional)
           (?:T (?=\d)                     # T, must be followed by a 
+digit.
           (?:(?!-) $RE{num}{int} H)?      # Non-negative integer, H (
+optional)
           (?:(?!-) $RE{num}{int} M)?      # Non-negative integer, M (
+optional)
           (?:(?!-) $RE{num}{decimal} S)?  # Non-negative decimal, S (
+optional)
           )?                              # Entire T part is optional
             /x;

while (<DATA>) {
    chomp;
    print "$_ ", /^$dur$/ ? "matches\n" : "does not match\n";
}

__DATA__
PnYnMnDTnHnMnS
-P1Y
P3Y3M3DT3H3M3S
P3M3Y3DT3H3M3S
P334Y3DT3H3M3S
PT2H
5T
PT
P5M6DT
P5M6DT9
P600T6S
P1347Y
P1347M
P1Y2MT2H
P0Y1347M
P0Y1347M0D
P-1347M
-P1347M
P1Y2MT
P1Y2M
P
PT0S
PT0.1234S
[download]

Running this results in:

PnYnMnDTnHnMnS does not match
-P1Y matches
P3Y3M3DT3H3M3S matches
P3M3Y3DT3H3M3S does not match
P334Y3DT3H3M3S matches
PT2H matches
5T does not match
PT does not match
P5M6DT does not match
P5M6DT9 does not match
P600T6S does not match
P1347Y matches
P1347M matches
P1Y2MT2H matches
P0Y1347M matches
P0Y1347M0D matches

P-1347M does not match
-P1347M matches
P1Y2MT does not match
P1Y2M matches
P does not match
PT0S matches
PT0.1234S matches
[download]

Abigail