The empty pattern //

If the PATTERN evaluates to the empty string, the last successfully matched regular expression is used instead. In this case, only the g and c flags on the empty pattern are honored; the other flags are taken from the original pattern. If no match has previously succeeded, this will (silently) act instead as a genuine empty pattern (which will always match).

That's why your third example works: /a/ matches in both alfa and whatever.

$ printf "Foo\nBar\nQuz\nBaz" | perl -nle 'print // ?"yes":"no"; /a/'
yes
yes
no
yes
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(See also Repeated Patterns Matching a Zero length Substring, although I think that's less relevant in this case.)