if i understood correctly, the matter is different:

$ cat t.pl
#!/usr/bin/perl
print "foo=$foo\n";
$ perl t.pl -foo=bar
foo=
[download]

and that's is expected.

$ cat t.pl
#!/usr/bin/perl -s
print "foo=$foo\n";
$ perl -s t.pl -foo=bar
foo=bar
[download]

and that's too, but:

$ cat t.pl
#!/usr/bin/perl -s
print "foo=$foo\n";
$ perl t.pl -foo=bar
foo=1
[download]

if i call it without -s, but the shebang say -s, perl assign 1 which is neither bar or undef

Oha

update: with the help of dada i think we have figured out it was a bug in perl.c function init_argv_symbols. checking on dev.perl.org on latest 5.x sources seems fixed btw. could anyone confirm? it's long since i read C code...