The fact that Result1 is a sigma proves that the shell script did not encode the \xe4

I disagree, because the `` has some implicit translations going on that you aren't controlling.

To test it, you need to capture the raw output bytes of the -e under test. Then you can compare them to the expected values.

I show an example test where I print \xe4\xe0 twice: the first time, without a binmode in the oneliner; the second time, with a binmode in the oneliner. You can see that the bytes that are output are different. You can test that those bytes match your expectations.

C:\Users\peter.jones\Downloads\TempData\perl>chcp
Active code page: 437
C:\Users\peter.jones\Downloads\TempData\perl>perl pm.pl

__SOURCE__
#!perl

use 5.012; # strict, //
use warnings;
use IPC::Open2;
use Test::More;

undef $\;
print "\n__SOURCE__\n";
seek \*DATA, 0, 0;
print for <DATA>;

$\ = "\n";
{
    my $pid = open2(my $ofh, my $ifh, 'perl', '-e', q("print qq(\xe4\xe0)"));
    binmode $ofh, ':raw';   # need to read from the open2 output file handle in raw mode, so you're looking at bytes, _not_ characters
    chomp(my $line = <$ofh>);
    print "without binmode, the high 8-bit characters pass through untranslated: ", unpack 'H*', $line;
    is $line, "\xE4\xE0", 'the bytes should be unedited';
    print "and printed out during test script: '$line'";
}
{
    my $pid = open2(my $ofh, my $ifh, 'perl', '-e', q("binmode STDOUT, ':encoding(Cp437)'; print qq(\xe4\xe0)"));
    binmode $ofh, ':raw';   # need to read from the open2 output file handle in raw mode, so you're looking at bytes, _not_ characters
    chomp(my $line = <$ofh>);
    print "with binmode, xE4 gets translated to x84 for a-umlaut, and xE0 gets translated to x85 for a-grave: ", unpack 'H*', $line;
    is $line, "\x84\x85", 'the bytes should be CP437-encoded';
    # so here, instead of printing the hexdump of the captured line, you could compare
    print "and printed out during test script: '$line'";
}
done_testing();

__END__
__OUTPUT__
without binmode, the high 8-bit characters pass through untranslated: e4e0
ok 1 - the bytes should be unedited
and printed out during test script: 'Σα'
with binmode, xE4 gets translated to x84 for a-umlaut, and xE0 gets translated to x85 for a-grave: 8485
ok 2 - the bytes should be CP437-encoded
and printed out during test script: 'да'
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