You can use a character class. What's a character class? It's a user-defined set of characters to match. For example, if you wanted your string to contain only characters between a and z, do this:

if ($string =~ /^[a-z]+$/) {
    # we have a string of at least one character of the range a-z
}
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You can also negate a character class with the ^ symbol at the start:

if ($string =~ /[^a-z]/) {
    # if it contains even one character NOT in the range, it doesn't m
+eet the criteria
    # do some error
}
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That's probably your best bet. If you didn't mind A-Z as well, you could use \W, which matches non-word (\w) characters (which are A-Za-z0-9_). For more gritty details, see perlman:perlre.