is there any reason to prefer using Win32::Socketpair over it?

It depends whether you're asking about the interface or the implementation.

I don't know what differences there might be in the implementations, but winsocketpair is obviously easier to use than socketpair.

my ($fd1, $fd2) = winsocketpair();
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vs

use Socket qw( AF_UNIX SOCK_STREAM PF_UNSPEC );
socketpair(my $fd1, my $fd2, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
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(Just to make things confusing, it has has to be a UNIX socket, not an INET socket.)

since perl 5.10, the socketpair builtin is also available on Windows

I was unaware of that.

is there any reason to prefer using Win32::Socketpair over it?

I mostly remember Win32::SocketPair for winopen2(). When I needed bi-di comms with a child process, I found that simply, effective piece of code worked reliably when IPC::Open2, (and the raft of huge & complicated 'portable' modules like IPC::Run with its gargantuan interface, multiple packages and "pump processes"), just hung me out to dry.

I say, because I have not been maintaining Win32::Socketpair for a long time, but maybe it is still useful.

I haven't used it for a while, I've only had the need for it once. But I've recommended it a few times without getting negative feedback, so if it ain't broke don't fix it :)

since perl 5.10, the socketpair builtin is also available on Windows