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if ($string =~ /[^A-Z0-9]/) {
   # String contains a character other than A-Z, 0-9.
...
   # String only contains characters A-Z, 0-9.
   ...
}

if ($string =~ /^[A-Z0-9]*\z/) {
   # String only contains characters A-Z, 0-9.
...
   # String contains a character other than A-Z, 0-9.
   ...
}