I ran the code for rack size 7. It took 12 hours 6 mins 18 secs on a 4 core Xeon box with 8 GB memory, OS linux. I am not going to copy the whole output here, just a few first and last lines:

MMVFWXQ => 6.24704795748769e-11
__BPMKQ => 6.24704795748769e-11
BCBMYHV => 6.24704795748769e-11

...

EEAIOND => 3.56231662727778e-05
EEAIONU => 3.56231662727778e-05
EEAIONL => 3.56231662727778e-05
Combinations: 8275164, Count: 16007560800.
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I probably have an error in the code :-)

Update: I see it now. The error was my effort to "DRY". The @tiles array must be defined as follows to make it work:

my @tiles = (('_') x 2,  # 2 blank tiles (scoring 0 points)
             # 1 point
             ('E') x 12,
             ('A') x 9, ('I') x 9,
             ('O') x 8,
             ('N') x 6, ('R') x 6, ('T') x 6,
             ('L') x 4, ('S') x 4, ('U') x 4,
             # 2 points
             ('D') x 4,
             ('G') x 3,
             # 3 points
             ('B') x 2, ('C') x 2, ('M') x 2, ('P') x 2,
             # 4 points
             ('F') x 2, ('H') x 2, ('V') x 2, ('W') x 2, ('Y') x 2,
             # 5 points
             ('K'),
             # 8 points
             qw(J X),
             # 10 points
             qw(Q Z));
[download]

Stay tuned, the next result is comming in 12 hours.