Win32::API's "P" will copy the "string" up to the first '\0'. It sounds like you have a (what Microsoft calls) "UNICODE" string (16-bit characters), so you've got something like "s\0t\0r\0i\0n\0g\0\0\0". So you can't use Win32::API's "P" since then only "s\0" gets copied.

But if that is the case, then you are probably supposed to pass in 16-bit-character strings as well. Which means something similar to (I can't really test this since I don't have your *.dll):

#!/usr/bin/perl -w
use strict;

use Win32::API;

my $PGetAddress = Win32::API->new(
        'IBKGetAddress2.dll', 'GetAddress', 'PP', 'N' )
    or  die "Can't import API : $!\n";

my $Address = pack "S*", unpack("C*","419 w 154"), 0;
my $ZipCode = pack "S*", unpack("C*","10032"), 0;

my $return = $PGetAddress->Call( $Address, $ZipCode )
    or  die "Can't run API: $! ($^E)\n";

my $string = pack "C*", unpack "S*", unpack "P2000", pack "L", $return
+;
$string =~ s/\0.*//s;

print "Output: ($string)\n";
[download]

If you've got a variable-length output string, then things get really ugly.

But this is all just guessing.

Oh, and by way of explanation, the first two strings of pack/unpack convert 8-bit-character strings into 16-bit-character strings. The last string of pack/unpack goes like this:

Make what looks like a C struct containing the pointer that was returned:
    pack "L", $return;
Copy into a Perl string the 2000 bytes which that pointer points to:
    unpack "P2000",
Get the list of numbers from treating each pair of bytes (16 bits) as a number:
    unpack "S*",
Build a string from those numbers:
    my $string = pack "C*",
Remove the terminating '\0' and anything after it:
    $string =~ s/\0.*//s;