I personally believe that quite a full overview of all the possibilities has already been offered to you. But there's a last one missing, that I'm reporting here FYI and for completeness: the implicit open associated with the magic ARGV filehandle is the 2-args one. (Which incidentally I regard as problematic in many senses...) Thus you can pass commands to your script on the command line:

spock:~ [18:59:27]$ perl -0777e 'my $v = <>; print $v' 'ls|'
Desktop
Mail
Mailbox
bin
dead.letter
...
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