When I need to know if a file has changed on me (Linux), I check the file size, device, inode and modified time. I don't usually need to check files for change, so I generally code it up from scratch as required.

For a change I thought I'd wrap it up into a sub and reuse it were needed. Many eyes will hopefully catch anything I've missed. This sub takes a filename, file handle or nothing (in which case the current stored value of stat is used) and returns a unique string representing the current state of the file. The same file (handle) will continue to return the same string until the file changes.

One obvious pitfall I can think of is someone could change the file contents (keeping the same size) then reset the mod time to match the original, but I'm ignoring that. It also occurs to me that this may not be accurate where links are involved.

Note:
I'm not trying to make it work in all environments (I'm pretty sure it won't work as expected in windows).
I'm not trying to positively test that the file contents have changed, only that they 'may' have changed.
I'm also trying to avoid calling in another module to do the job.

# pass in a file name, a file handle or nothing
# returns a unique string that changes when the file changes
sub FileMeta {
    if (my $fh = shift) {
        my ($dev, $inode, $size, $mtime) = (stat($fh))[0,1,7,9];
        return "$size:$dev;$inode:$mtime";
    }
    else {
        my ($dev, $inode, $size, $mtime) = (stat(_))[0,1,7,9];
        return "$size:$dev;$inode:$mtime";
    }
}
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