First, you have bad syntax in your elsif conditional, a single = which assigns $a the value of 2. It should be ==.

I reformatted your code to be more readable, fixed the ==, added 'my' to the subroutine vars, removed the 'v' filehandle so I could just print to the screen, seems to work for me based on your logic. Here is the code (always 'use strict;'!).

#!/usr/bin/perl -w

use strict;
$| = 1;

my $a = 1;
my $b = 1;
my $c = 1;
print "Passing in \$a: $a, \$b: $b, \$c: $c: expect p 1 1\n";
ddd($a,$b,$c);

$a = 2; 
$b = 2; 
$c = 2; 
print "Passing in \$a: $a, \$b: $b, \$c: $c: expect pp 2 2\n";
ddd($a,$b,$c);

$a = 3; 
$b = 3; 
$c = 3; 
print "Passing in \$a: $a, \$b: $b, \$c: $c: expect 3 w 3\n";
ddd($a,$b,$c);

sub ddd {

    my $a = $_[0];
    my $b = $_[1];
    my $c = $_[2];

    if ($a == '1') { 
        $a = 'p'; 
    }
    elsif ($a == '2') { 
        $a = 'pp'; 
    }

    if ($b == '3') { 
        $b = 'w'
    }

    print "  < $a >  < $b > < $c > \n";
}
[download]

and here is the output:

#> ./test-subroutine.pl
Passing in $a: 1, $b: 1, $c: 1: expect p 1 1
  < p >  < 1 > < 1 > 
Passing in $a: 2, $b: 2, $c: 2: expect pp 2 2
  < pp >  < 2 > < 2 > 
Passing in $a: 3, $b: 3, $c: 3: expect 3 w 3
  < 3 >  < w > < 3 > 
#>
[download]