Each /e tacked onto the end is essentially doing another eval. The first eval evaluates $1, which gets it what was matched -- namely, the whole string. The second /e applies eval to what we just got -- the whole string. Thus, it evaluates whatever you typed.

As a side note, you can have your cake and eat it two with your second option -- perl -ple "$_=eval" will give you your newline.

perl -pe '"I lo*`+$^X$\"$]!$/"=~m%(.*)%s;$_=$1;y^`+*^e v^#$&V"+@( NO CARRIER'