In your example the array is in scalar context (due to concatenation). An array in scalar context returns the number of elements in the array.

The efficiency bit is referring to:

print 'hello', 'bob', 'smith', "\n";
#vs.
print 'hello' . 'bob' . 'smith' . "\n";
[download]

The second one requires perl to do more work as it must do a concatenation on all the elements and then print does it's thing on the result. The former works because print works in list context by simply doing it's thing to each and every element of the list you provide. See $OFS in perlvar and print for some other differences.

--
perl -pe "s/\b;([st])/'\1/mg"