Well then here's how I'd do it. I'd check the whole file for UTF-8 sequences and any other bytes with value 128 or above.

• If you find no bytes with value 128-255, then the file is ASCII (or CP-1252 or UTF-8, they're all the same here.)
• If you only find valid UTF-8 byte sequences then it's probably UTF-8. (If the first sequence is at the start of the file and it's a BOM character, value 0xFEFF, then there is very little doubt about it)
• If you only find other upper half bytes then it's CP-1252.
• If you find both, it's more likely that it's CP-1252, but you'd better take a look at it; It could be a corrupt UTF-8 file.

Code to test this, assuming $_ contains the whole file, and is not converted to utf-8:

my(%utf8, %single);
while(/([\xC0-\xDF][\x80-\xBF]|[\xE0-\xEF][\x80-\xBF][\x80-\xBF]|[\xF0
+-\xF7][\x80-\xBF][\x80-\xBF][\x80-\xBF])|([\x80-\xFF])/g) {
    if($1) {
        $utf8{$1}++;
    } elsif($2) {
        $single{$1}++;
    }
}
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(untested)

If after this code block %single is empty and %utf8 is not empty, then it's UTF-8; if %single is not empty then it's CP-1252 with high certainty if %utf8 is empty. <You can do simpler tests than this one, that don't involve hashes, but this way it's easier to debug and verify why it decided one way, and not another way.