# assuming; ...
my $str = "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";

sub replace {
	my ($extracted_number) = @_;

	# calculate or query the new number. For example;
	my $new_number = $extracted_number + 1;

	return $new_number;
}

$str =~ s/
	(?<=,)			# Precondition: There should be a comma before,
	\s*
	(\d+\.\d+)		# ... and it should be a floating number!
/replace($1)/xges;

print "$str\n";