Unmatched ) in regex; marked by <-- HERE in m/
    [A-Z]+               # WB
    \(\d+,\d+) <-- HERE            # (1,2)
    \s*=\s*              # =
    (?:                  # start non-capturing group
       \d+\.\d+,         # a float followed by comma
       \s*(\d+\.\d+),?   # capture a float (followed by comma?)
    )                    # end-group
  / at /tmp/wb line 10.

##</code><code>##

$_ = 'WB(1,2)=    0.000, 1.23, TB(1,2)=     0.0,  253.0, TMB(1,2)=  0.0,  1.0,  SL(1,2)= 0.00, 1.00';
my @parts = split /, /;
$parts[1] = 3.21; # change 1.23 with 3.21
$_ = join ', ', @parts;
print $_, "\n";

# output:
# WB(1,2)=    0.000, 3.21, TB(1,2)=     0.0,  253.0, TMB(1,2)=  0.0,  1.0,  SL(1,2)= 0.00, 1.00