$_ = '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