Firstly, please use <code> and </code> tags.

You could set up your @alphabet using the range operator (..), like this

my @alphabet = (q{a} .. q{z});
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You probably want to remove the newline (or whatever your platform uses) before doing any comparison

chomp(my $hladane = <STDIN>);
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You are not assigning the first letter of the alphabet to $slovo but doing the reverse, change to

$slovo = $alphabet[$nummer];
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Perl does allow you to increment letters

$ perl -le '$let = q{a}; $let ++; print $let;'
b
$
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I hope this is of use.

Cheers,

JohnGG