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

if ($string =~ s!^(\w{4})$!uc($1)!e) {
    
    # do something
...
    # do some error
    # print "Location: $error_url"
}