Has something happened?

Nope.

Why is while (<>) not doing what it use to?

It's doing the same thing as it has since at least 1999, when I started using Perl, and probably always has.

How do I get the old behavior

<> reads either from files named as command-line arguments or from STDIN. It does not, and never has, returned the literal command-line arguments themselves.

If you want to access the literal command-line arguments, you can find them in the array @ARGV. e.g.:

$ perl -E 'for (@ARGV) { say $_ }' foo bar baz
foo
bar
baz
[download]