It looks to me like the regex is getting confused when it is backtracking. As BioLion notes above, jwrahn's explanation fits the output perfectly if we remove the \s* between each letter and digit, but it doesn't fit the output when the \s* is still in place.

while (<main::DATA>)
{
  chomp;
  print "\nTesting '$_'\n";
  /^(?:(?:(\d+)c\s*)|(?:(\d+)w\s*)|(?:(\d+)r\s*))+/i;
  print "Without \\s*    :   1='$1', 2='$2', 3='$3'\n";
  /^(?:(?:(\d+)\s*c\s*)|(?:(\d+)\s*w\s*)|(?:(\d+)\s*r\s*))+/i;
  print "With \\s*       :   1='$1', 2='$2', 3='$3'\n";
}
[download]

outputs

Testing '1c'
Without \s*    :   1='1', 2='', 3=''
With \s*       :   1='1', 2='', 3=''

Testing '2w'
Without \s*    :   1='', 2='2', 3=''
With \s*       :   1='', 2='2', 3=''

Testing '2c3w'
Without \s*    :   1='2', 2='3', 3=''
With \s*       :   1='3', 2='3', 3=''

Testing '1w1w'
Without \s*    :   1='', 2='1', 3=''
With \s*       :   1='1', 2='1', 3=''

Testing '1w2r'
Without \s*    :   1='', 2='1', 3='2'
With \s*       :   1='2', 2='2', 3='2'

Testing '2r1c'
Without \s*    :   1='1', 2='', 3='2'
With \s*       :   1='1', 2='', 3='2'
[download]

Best, beth