Note that if you're only dealing with letters, you can use ++ on strings.

$ perl -e'$g="a"; print $g++, " " while $g ne 'cw'; print "\n"'
a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af 
+ag ah ai aj ak al am an ao ap aq ar as at au av aw ax ay az ba bb bc 
+bd be bf bg bh bi bj bk bl bm bn bo bp bq br bs bt bu bv bw bx by bz 
+ca cb cc cd ce cf cg ch ci cj ck cl cm cn co cp cq cr cs ct cu cv
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Otherwise, your mapping is backwards. You should be working with numbers (indexes into @mapping), then mapping to the characters.

my @mapping = ('a'..'z');

my $guess = join '', @mapping[ @guess ];
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It might be a bit faster to build $guess relative to the previous value rather than building it from scratch every time, but that's easy to change later.