The diamond operator <> attempts to read from the file denoted by the elements in @ARGV (it tries reading from $ARGV[0] and increases the index in case opening the file denoted by the previous index fails).
An assignment of the form @ARGV=$0 is the same as @ARGV=($0), which actually means $ARGV[0]=$0. Since $0 is the filepath of the perl script itself, the diamond operator will read the first line from the perl script file.
The diamond operator will read from STDIN only if the @ARGV list is empty. | [reply]
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