C++, Added 194, changed 2

Honest solution in 1 file.

#include <iostream>
#define readln(a) std::cin>>a
#define writeln(a) std::cout << a
#define var int
#define program int main(){
#define test 
struct E{
    int n;
} end;
#define integer 
#define begin 
 
program test;
var a, b, c integer;
begin
    readln(a);
    readln(b);
    c = a + b;
    writeln(c);
end.
 
n;
}

Perl, preparation (423+17) 440, modified 0

Plug-in module PascalSyntax.pm:

package PascalSyntax;
use Filter::Simple;
use Exporter;
@ISA=(Exporter);
@EXPORT = qw(var program readln writeln);
sub var(@){$_=undef for @_}
sub readln(@) {chomp($_[0]=<>);}
sub writeln(@) {print @_;}
sub program(@) {};
FILTER {
 s/:=/=/g;s/:\s+\w+//g;
 s/begin/{/g;
 s/end\.?/}/g;
 for(/var\s+(.*?);/) {
  my $t=$1; $t=~s/\s+//g;
  for (split(/,/,$t)) {
   my $var=undef;
   *{'main::'.$_}=sub():lvalue{$var};
  }
 }
};

Program:

#!/usr/bin/perl
use PascalSyntax;

program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.

In this case, knowing the flexibility of perl, I wanted the code to work on its own, with minimal changes. And this is despite the fact that in perl, all variables must begin with the sign $. What was done:

1. The Filter module is applied to the code, which allows you to change it before the compilation stage. It replaces begin and end with curly brackets (you could not to do this by declaring functions with these names). And they are deleted : because I did not find any other ways to use them (there is no "colon" operator in perl and it cannot be overloaded).
2. The same filter parses the string var and creates functions with variable names in the main program. Returning each its own variable created in the closure. Functions have the attribute lvalue, which allows you to assign values to the variables they return (the construction c()=a()+b() becomes working, and parentheses for the functions known at the time of compilation are not required)
3. Functions are created for the other pascal keywords (program, var, read/writeln).

JavaScript, preparation (without formatting 223+5) 228, Changed: 0

The program text can still be placed in a comment, without violating the restrictions on the task, if this text can then be extracted and executed. This solution applies this approach. The comment was used because there is no here-document syntax for strings in JavaScript.

What's going on:

• The entire function text, including comments, can be converted to a string in JavaScript. Through f.toString() we get the text of the function, in the body of which there is a comment with the program.
• The first replace deletes the first 4 and last 2 rows.
• The second replace wraps the arguments for readln and writeln in quotation marks. It uses the fact that c also falls into window as a global variable.
• The third removes the colon.

You can view here. I checked it on Firefox, Chrome.

Code with formatting:

(function (f) {
    var readln = function (x) {
        window[x] = +prompt()
    };
    var writeln = function (x) {
        alert(window[x])
    };
    eval(f.toString()
        .replace(/(.+\n){4}([\s\S]+)en[^]+$/, '$2')
        .replace(/\((.)\)/g, '("$1")')
        .replace(':', '')
    )
})(function(){/*
program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.
*/});

C, preparation 157, modified 1

C, preparation 159, 162, 187, 196, modified 1.

We managed to get rid of one colon by using bit fields in the structure. I couldn't think of anything for the second colon. :)

Code here.

#define test
#define var struct{int
#define integer 1;}end;a,b,c
#define begin main(){
#define readln(x) scanf("%d",&x)
#define writeln(x) printf("%d",x)

program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c = a + b;
    writeln(c);
end.

a;}

After processing by the preprocessor:

# 1 "test.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "test.c"

program;
struct{int a, b, c: 1;}end;a,b,c;
main(){
    scanf("%d",&a);
    scanf("%d",&b);
    c = a + b;
    printf("%d",c);
end.

a;}

C#, Preparing 804, Modified 0

It is not written optimally. It won't start on Mono. But you can add more" known " constructs and change the expression. Not an ideal solution for the contest :)

using System;
using System.Data;
using System.Text.RegularExpressions;
using VarIntegers = System.Collections.Generic.Dictionary<string, int>;

namespace Q526265_MyAnswer
{
    class Program
    {
        static void RunProgram(string textProgram)
        {
            DataTable dt = new DataTable();
            VarIntegers vars = new VarIntegers();
            Func<string, string> getVarName = str => Regex.Match(str, @"\(([\w]+)\)").Groups[1].Value;
            foreach (var item in textProgram.Replace("\r\n", "").Split(';'))
            {
                if (item.Contains("readln"))
                    vars[getVarName(item)] = int.Parse(Console.ReadLine());
                else if (item.Contains("writeln"))
                    Console.WriteLine(vars[getVarName(item)]);
                else if (item.Contains(":="))
                {
                    var exp = Regex.Split(item.Replace(" ", ""), ":=");
                    vars[exp[0]] = (int)dt.Compute(Regex.Replace(exp[1], @"[\w]+", m => vars[m.ToString()].ToString()), "");
                }
            }
        }

        static void Main()
        {

            RunProgram(@"
program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.
");
            RunProgram(@"
program test;
var number1, number2, number3: integer;
begin
    readln(number1);
    readln(number2);
    number3 := number1 * number2 - (number1 + number2);
    writeln(number3);
end.
");
        }
    }

}

Javascript ES6, added 100, changed 0

readln=prompt,writeln=alert;eval(`
program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.
`.replace(/^[pbe].*|: ?\w*/gm,"").replace(/\w+\((.)\)/g,"$1=+$&"))

C++, added 252, changed 1 (and it's not a colon!)

Https://ideone.com/WxCd1s

#include <iostream>

#define program int
#define test main(){
#define var int
#define v(t) integer=a+b;u##t
#define z(t) v(t)
#define c z(__LINE__)
#define begin
#define readln std::cin>>
#define writeln(x) std::cout<<integer
#define end std::cin

program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c : a + b;
    writeln(c);
end.

get();}

Typescript, added 127 120, changed 2

Sandbox

declare type integer=any
~function(a,b,writeln,end){var test,begin,u=arguments,readln=i=>u[i]=+prompt(),

program=test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c = a + b;
    writeln(c);
end.

a}(0,1,alert,{})

Javascript ES6, added 93 86, changed 3

Similar idea, but with multiple global variables:

~function(a,b,writeln,end,test,begin){readln=i=>arguments[i]=+prompt()
  
program=test;
var a, b, c, integer;
begin
    readln(a);
    readln(b);
    c = a + b;
    writeln(c);
end.

a}(0,1,alert,{})

The default value cannot be used, because it breaks the connection between arguments and the names.

C, Preparation: 180, changed 5

#define program
#define test
#define begin int main(void){
#define end }
#define var(a,b,c,...) int a,b,c
#define readln(a) scanf_s("%d",&a)
#define writeln(a) printf("%d",a)

program test;
var(a, b, c, : integer);
begin
readln(a);
readln(b);
c = a + b;
writeln(c);
end

C#

Another option, here the pascal code is already translated into c# code, which is then compiled and run.

using System;
using System.Reflection;
using System.Text.RegularExpressions;

namespace Q526265_MyAnswer2
{
    class Program
    {
        static void RunProgram(string pascalProgram)
        {
            string className = "Program";
            Func<string, string> pascalType2SharpType = pascalType =>
            {
                switch (pascalType)
                {
                    case "integer":
                        return "int";
                }
                return "object";
            };
            var sharpProgram = Regex.Replace(pascalProgram, @"program (\w+);", m => { className = m.Groups[1].Value; return m.Result(@"
                using System;
                class $1 {
                    static void readln(ref int variable) {
                        variable = int.Parse(Console.ReadLine());
                    }
                    static void writeln(int value) {
                        Console.WriteLine(value);
                    }
            "); });
            sharpProgram = Regex.Replace(sharpProgram, @"var (.*): (\w+);", m => m.Result("static " + pascalType2SharpType(m.Groups[2].Value) + " $1;"));
            sharpProgram = Regex.Replace(sharpProgram, @"readln\((\w+)\)", "readln(ref $1)");
            sharpProgram = sharpProgram.Replace("begin", "static void Main() {").Replace("end.", "}}").Replace(":=", "=");

            object programObject = new Microsoft.CSharp.CSharpCodeProvider()
                .CompileAssemblyFromSource(new System.CodeDom.Compiler.CompilerParameters(), sharpProgram)
                .CompiledAssembly.CreateInstance(className);
            programObject.GetType().GetMethod("Main", BindingFlags.Static | BindingFlags.NonPublic).Invoke(null, null);
        }

        static void Main(string[] args)
        {
            RunProgram(@"
program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.
");
            RunProgram(@"
program MyProgram;
var a, b, c, d: integer;
begin
    readln(a);
    readln(b);
    readln(d);
    c := (a + b) / d;
    writeln(c);
end.
");    
        }
    }
}

Https://ideone.com/Dt0WhE

Shorter version: https://ideone.com/jwdByx

Python3, preparing 423, modified 0

code_pas = '''\
program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.'''

code_pas = code_pas.split('\n')
code_pas = code_pas[3:7]
code_pas[0] = code_pas[0].replace('    readln(a);','a = int(input())')
code_pas[1] = code_pas[1].replace('    readln(b);','b = int(input())')
code_pas[2] = code_pas[2].replace('    c := a + b;','c = a +b')
code_pas[3] = code_pas[3].replace('    writeln(c);','print(c)')
code = '\n'.join(code_pas)
co = compile(code,'','exec')
exec(co)

Http://ideone.com/fGsz4n

Out-of-competition option:

code_pas = '''\
program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.'''
code_py = '''\
a = int(input())
b = int(input())
c = a + b
print(c)'''

code = code_pas.replace(code_pas, code_py)
co = compile(code,'','exec')
exec(co)

Http://ideone.com/L4ilJD

Java, Preparation: >1500 (I can't count exactly) 1500, Changed: 0

A small example of translation from Pascal to Java and then executing the result on the fly using the BeanShell library.

I think that it is not written perfectly, because it does not have enough flexibility.

A special feature of the solution is that the Pascal syntax is also slightly preserved in the resulting Java code.

The Engine class (implements the required functionality):

import bsh.EvalError;
import java.io.IOException;
import bsh.Interpreter;

public class Engine {

//Задание с ru.SO, скопированное без каких-либо изменений
private final String sourcePascal = "program test;\nvar a, b, c: integer;\nbegin\n    readln(a);\n    readln(b);\n    c := a + b;\n    writeln(c);\nend.";

private final String classPrefix = "public class Test{";
private String methodPrefix = "public void testMethod(){";
private final String postfix = "}";
private final String readln = "private int readln(){Scanner in = new Scanner(System.in);return in.nextInt();}";
private final String writeln = "private void writeln(int a){System.out.println(a);}";
private final String engineScript = "Test t = new Test();t.testMethod();";

private String translateToJava(String source) {
    String preparedCode = source.substring(source.indexOf(";") + 1).replace(":=", "=").replace("var a, b, c: integer", "int a,b,c").replace("begin", "{").replace("end.", "}").replace("readln(a)", "a=readln()").replace("readln(b)", "b=readln()");
    String methodScript = methodPrefix +preparedCode+ postfix;
    String classScript = classPrefix + readln + writeln + methodScript + postfix;
    String result = classScript + engineScript;
    return result;
}

public void run() throws IOException, EvalError {        
    Interpreter i = new Interpreter();
    i.eval(translateToJava(sourcePascal));
}
}

Using the Engine class:

Engine en =new Engine();
en.run();

The resulting Java code that the Engine class mechanisms turn a Pascal task into:

public class Test{private int readln(){Scanner in = new Scanner(System.in);return in.nextInt();}private void writeln(int a){System.out.println(a);}public void testMethod(){
int a,b,c;
{
    a=readln();
    b=readln();
    c = a + b;
    writeln(c);
}}}Test t = new Test();t.testMethod();

Javascript ES6, added 86 79, changed 3 (without eval)

begin=end=test=writeln=alert;with(o={a:'a',b:'b',readln(x){o[x]=+prompt()}}){

program=test;
var a, b, c, integer;
begin
    readln(a);
    readln(b);
    c = a + b;
    writeln(c);
end.

a}

Python3:

code = """\
program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.
"""\
.replace('program test;', '')\
.replace('var a, b, c: integer;', '')\
.replace('begin', '')\
.replace('end.', '')\
.replace('    ', '')\
.replace(':=', '=')\
.replace('writeln', 'print')\
.strip()

import re
code = re.sub('readln\((.+)\);', r'\1 = int(input())', code)

exec(code)

Replacing operands / operators with similar ones. To replace readln, a regular expression is used.

Perl, Added 304, Changed 0

p('
program test;

var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.
');

sub p {
    # Получаем токены.
    while (my $s = substr($_[0], $i++, 1)) {
        $s !~ /\s/ ? $t .= $s : $t ne '' ? ((push @t, $t) and ($t='')):();
    }

    # Анализируем их, составляя текстовый аналог pascal программы для perl.
    for (my $i=0; $i < @t; $i++) {
        $e .= '$'.substr($t[$i],7,1).'=<>;' if $t[$i]=~/^readln/;
        $e .= '$'.$t[$i-1].'=$'.$t[$i+1].$t[$i+2].'$'.$t[$i+3] if $t[$i] eq ':=';  
        $e .= 'print$'. (substr($t[$i], 8, 1)).';' if $t[$i]=~/^writeln/;
    }

    # Выполняем программу.
    eval $e;
}

Sh/S, Preparation: ~175, Changed:0

I don't know how fair it is or how to count it.

echo '
Dreadln(a) scanf("%d",&a);
Dwriteln(a) printf("%d",a);
Dvar int
Dtest
Dinteger
Dbegin main(){
Dend ;
program test;
var a, b, c: integer;
begin
    readln(a);
    readln(b);
    c := a + b;
    writeln(c);
end.
}' | sed 's/[.:]//g' | sed 's/D/#define /g'| gcc -x c - -o 1 && ./1

The header parameters are approximate, perhaps you can still improve. Thanks to chatik for the ideas.