Actually the OP's open:

open (FILE,$file) || die ("cant open file");
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is correct, but old school and frowned on. As you suggest the three parameter version of open is better, but even better is to use it with a lexical file handle. Using a lexical file handle ensures the file is closed when the file handle variable goes out of scope. The improved version of open is:

open my $fileIn, "<", $file or die "Can't open $file: $!";
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Note too the file name mentioned in the die and the use of $! to give the system provided failure message. Both of those can help a lot when figuring out why an open failed.