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If you have to generate a sequence of letters, this will do:

   my @alphabeth = 'a' .. 'z';
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You never initialise $slovo, but you keep using it, expecially in the while test. Moreover, are you sure that you get an ERROR and not a WARNING?

If you want to increment a letter, it's as easy as doing this:

   my $letter = 'p';
   $letter++; # Now $letter is 'q'
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Note that you should chomp immediately when you get the input letter, something along the lines:

   chomp(my $hladane = <STDIN>);
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This also means that you don't need to chomp() in the while loop.

Regarding the newline problem, note that you never chomp() the $lf variable, that's where you get your newline from.