The reason is structure alignment (i.e. how the structure is stored in memory). I'd guess that it gets rounded up to the nearest DWORD boundary.

Running your snippet with $Win32::API::DEBUG=1 reveals some detail about what's going on:

use Win32::API;
use Win32::API::Struct;
$Win32::API::DEBUG = 1;

Win32::API::Struct->typedef( test => qw(char i; long l;) );
my $b = Win32::API::Struct->new( 'test' );
print $b->sizeof;
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Output:

(PM)Struct::recognize got 'i', 'char' -> 'c'
(PM)Struct::recognize got 'l', 'long' -> 'l'
(PM)Struct::new: got 'test'
(PM)Struct::sizeof: sizeof with member(i) now = 1
(PM)Struct::sizeof: sizeof with member(l) now = 5
(PM)Struct::sizeof first=1 align=4
(PM)Struct::sizeof returning 8
8
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