... why 'a', 'd', 'f' and 's' are so special?

Because, e.g.,  q aasdfa includes the delimiter in the delimited string.

c:\@Work\Perl\monks>perl -wMstrict -lE
"say q abcda;
 say q defgd;
 say q fghif;
 say q stuvs;
"
bcd
efg
ghi
tuv
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