I can see what the OS passes to perl as "char *argv[]", and also what Perl actually stores in @ARGV, with the following: 
# x.pl
use v5.28; use Data::HexDump;
use Encode qw(encode decode);
my $a0 = $ARGV[0];
my $a0d = decode( q(UTF-8), $a0 );
say HexDump($a0);
say HexDump($a0d);
open my $fh, q(>), $a0;
open my $fhd, q(>), $a0d;
sleep 99;

# shell
aa=$(perl -CSDA -we 'print 1 . chr(0xb1) . chr(0x155) . 2')
perl -w x.pl "$aa" &
ls -rt
xxd /proc/`pidof perl`/cmdline

[download]

This also creates a file (or two) with a "funny" name.

Now, on my particular system, it seems that perl starts with a UTF-8 encoding of "$aa" in its /proc/cmdline (i.e. the C "char *argv[]"), and that Perl @ARGV corresponds verbatim to C argv[].

It also seems that for open(), both byte-array strings, and also strings with one unicode codepoint / char (as obtained via decode) create the same filename. Maybe Perl does an implicit utf8 decode()?

But this is just an experiment on a particular setup. I'm not really sure (1) what Perl actually does on startup (does it process its C argv, maybe according to locale), (2) what open() is supposed to take, and (3) what readlink(), readdir() etc are supposed to return. What are the rules?

EDIT: fixed second perl invocation

In reply to how are ARGV and filename strings represented? by almr

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