Empty Regex never fails but failed?

leszekdubiel has asked for the wisdom of the Perl Monks concerning the following question:

I am completely confused why this fails:

printf "alfa\nwhatever" | perl -CSDA -n -e '// or die "not possible! -
+- $_ --"; /alfa/;'
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Die should never happen because empty pattern matches any text... Why? Why? Same on 5.24 and 5.26. Linux Debian. The code

printf "alfa\nwhatever" | perl -CSDA -n -e '// or die "not possible! -
+- $_ --"; /al/;'
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also fails, but this one not (!!!!):

printf "alfa\nwhatever" | perl -CSDA -n -e '// or die "not possible! -
+- $_ --"; /a/;'
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