According to the "Useful tips" section (on page 7) of the official specs, you should be able to read the first 11 bytes of the file and match this pattern: 
/\xff.\xff...JFIF\x0/

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Based on looking at a small number of "*.jpg" files I happen to have uploaded from a camera (via "iPhoto", which may have been involved in "updating" some of those pictures after the upload), I would actually change that to: 
/\xff.\xff...(?:JFIF|Exif)\x0/

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And apparently, you might expect "JFXX" as well. But frankly, I'd be content to trust the file name, and so would focus on the first reply above.

UPDATE: The above assumes that the file is being read in :raw mode (what we commonly understand as the default behavior of doing binmode FH). Also, I agree with what afoken says below: use a module to validate jpeg files, in case there is any doubt about their validity.

In reply to Re: Need a RegExp for Images by graff
in thread Need a RegExp for Images by BlenderHead

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