Passing Variable from user defined function is done by using the values declared outside a function, make the neccessary calculation inside the function and then return the result whenever the function is called.If you are passing variables to a function there is an array @_ which lists all the variables when the function is called, to improve the usability of a function.

    #! C:\programfiles\perl\bin\perl
    print "content-type: text/html\n\n";
    $x = 12;
    $y = 13;
    $sum = add($x, $y);
    print "The SUM is:",$sum;
    sub add
     {
      my ($a, $b) = @_;
      $c = $a + $b;
      return $c;
     }

    The SUM is:25

In the above passing variable example the values of variables "$x=12","$y=13" is passed to the values "$a","$b" to do addition inside the function to store the values in "$c", the output is go outside the loop in the variable "$sum".