what other part should take care of the redirection 2>&1?

Perl itself?  Like it does on Unix when 2>&1 is the only shell meta characters in the command.^*

I don't think Perl is making this optimisation on Windows, though, but I do think it's a valid question — considering that in Perl things are not always quite as obvious as it might seem at first...

___

^* as you can easily verify:

$ strace -fqeexecve perl -e 'open FH, "echo foo bar 2>&1 |"; print <FH
+>'
execve("/usr/bin/perl", ["perl", "-e", "open FH, \"echo foo bar 2>&1 |
+\"; "...], [/* 77 vars */]) = 0
[pid  1819] execve("/bin/echo", ["echo", "foo", "bar"], [/* 77 vars */
+]) = 0
foo bar
[download]

No shell involved here.  But as soon as you add another shell metacharacter, e.g. ";", Perl will use the shell:

$ strace -fqeexecve perl -e 'open FH, "echo foo bar ; 2>&1 |"; print <
+FH>'
execve("/usr/bin/perl", ["perl", "-e", "open FH, \"echo foo bar ; 2>&1
+ |\""...], [/* 77 vars */]) = 0
[pid  1833] execve("/bin/sh", ["sh", "-c", "echo foo bar ; 2>&1"], [/*
+ 77 vars */]) = 0
foo bar
[download]