To clarify that a word containing an 'a' but no 'e' should be printed, I've added an extra "a" word to your test data. Your program:

for('a','anyone','cancel','declare','perlmonks'){$i=()=/e/g;print"$_: 
+$i\n"if(/a/);}
[download]

and mine:

/a/&&print"$_: ".y/e//.$/for'a','anyone','cancel','declare','perlmonks
+'
[download]

both print the identical output namely:

a: 0
anyone: 1
cancel: 1
declare: 2
[download]

but mine is 13 characters shorter.

For golf problems of this type I prefer to put the data not in code, but in a separate file read by the program from stdin. For example, for a data file t1.txt containing:

a
anyone
cancel
declare
perlmonks
[download]

we can run:

perl -lne"/a/&&print$_.': '.y/e//" t1.txt
[download]

to produce the same output.

Update: Just saw LanX -E switch update. For modern versions of perl, we can shorten to:

perl -lnE"/a/&&say$_.': '.y/e//" t1.txt
[download]

which should work everywhere. For Unix only, we can save another stroke or two by using -lnE'...' instead of -lnE"...".