However even this explanation doesn't satisfy completely: If indeed no redirection is performed by Perl, the called process should see the arguments ">...", "2>&1" in @ARGV, but they are not present. Something is eating them without doing proper redirection...

No. Consider

#!/usr/bin/perl --

unlink 'out.txt';
system 1, $^X, '-MData::Dumper', '-le','print(1);warn(2);print Dumper(
+\@ARGV);', '>out.txt 2>&1';
sleep 1;
print `cat out.txt`, "\n",'-'x66,"\n";
sleep 1;

print `pslist |grep -c cmd`,"\n",'-'x66,"\n";
system 1, 'perl.exe -MData::Dumper -le "print(1);warn(2);print `pslist
+ |grep -c cmd`, Dumper(\@ARGV)" >out.txt 2>&1';
sleep 1;
print `cat out.txt`,  "\n",'-'x66,"\n";

__END__

1
2 at -e line 1.
$VAR1 = [
          '>out.txt 2>&1'
        ];

cat: out.txt: No such file or directory

------------------------------------------------------------------

pslist v1.28 - Sysinternals PsList
Copyright ¬ 2000-2004 Mark Russinovich
Sysinternals

8

------------------------------------------------------------------
2 at -e line 1.
1

pslist v1.28 - Sysinternals PsList
Copyright ¬ 2000-2004 Mark Russinovich
Sysinternals

9
$VAR1 = [];


------------------------------------------------------------------
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After the 2nd system there is one more cmd process, and @ARGV no longer contains out.txt, ie it is the shell that does redirection.

You can use Proc::Background to launch processes in the background