Your question makes no sense. It doesn't return a char*, so you can't use char*. If you tell it it's a char*, Win32::API will treat it as a null-terminated string. Win32::API doesn't know anything about BSTR, so you'll have to tell Win32::API that the pointer is a number, fetch the BSTR yourself, and extract the string therein yourself.

use Encode     qw( decode );
use Win32::API qw( );

sub bstr_to_str {
   my ($bstr) = @_;
   my $len = unpack('V', unpack('P4', pack('J', $bstr-4)));
   my $raw = unpack("P$len", pack('J', $bstr));
   return decode('UTF-16le', $raw);
}

my $SysFreeString = Win32::API->new('oleaut32.dll', 'SysFreeString', '
+N', 'V')
   or die $^E;

sub SysFreeString {
   my ($bstr) = @_;
   $SysFreeString->Call($bstr);
}

my $dec = Win32::API->new('mydll', 'dec', 'IP', 'N')
   or die $^E;

sub dec {
   my  $iSize                   =  $_[0];
   our $iReturn; local *iReturn = \$_[1];

   #my $iReturn_buf = pack('l', $iReturn);  # If in-out
   my $iReturn_buf = pack('l', 0);          # If out

   my $rv_bstr = $dec->Call($iSize, $iReturn_buf);

   $iReturn = unpack('l', $iReturn_buf);
   
   my $rv = bstr_to_str($rv_bstr);
   SysFreeString($rv_bstr);

   return $rv;
}
[download]

Untested.

Notes:

• You didn't specify if iReturn is an input and an output, or just an output. I assumed the latter, but provided the code for the former in a comment.
• I kept the C calling convention (2nd argument is modified) since I didn't know enough about the function to make it more Perlish.
• I assumed the returned BSTR needs to be freed, and that this should be done using SysFreeString.

Update: Oddly, the pointer doesn't point to the start of the structure but to the start of the character data (for compatibility with WCHAR*). Fixed.