Thank you for taking the time and replying! I tried using say, but I have never done it before, so I didn't succeed that way. And I knew the substitution (s) operator returned by default the amount of substitutions made, but I couldn't use it on a read-only value, so tr was the asnwer, but I didn't know it existed! Always good to learn something new.
Your final code was

perl -E'/a/&&say"$_ :".y/e//for qw/anyone cancel declare perlmonks/'

Which is great, and already shorter than mine, but I was able to get it even shorter!

perl -E'/a/&&say"$_ :".y/e//for anyone,cancel,declare,perlmonks'

And reading from STDIN is a good idea, (and shortens the code even furter), but I wanted to get a one-liner that can just be copy-pasted in itself and does the job, without having to type anything.

Cheers, lunix