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if ($string =~ m!(^[A-Za-z0-9]{4})$!) {
    $string =~ tr/[a-z]/[A-Z]/;
...
} else  {
    # do some error
}

Benchmark: timing 1000000 iterations of tr, uc...
        tr: 10 secs ( 7.49 usr  0.00 sys =  7.49 cpu)
        uc:  9 secs ( 6.60 usr  0.00 sys =  6.60 cpu)