The easiest way I can see to do it would be taking the random number you get and using it as an index into an array of letters. Something that looks like the following:

@letters = ('a'..'z');    # or ('A'..'Z') or ('a'..'z', 'A'..'Z'), etc
+.
$letter = $letters[ int rand scalar @letters ];
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Which could be further condensed down if you only wanted to use this method once and wanted to make your code a little harder to read.