See I/O Operators in perlop, about the ninth paragraph:

The filehandles STDIN, STDOUT, and STDERR are predefined. (The filehandles "stdin", "stdout", and "stderr" will also work except in packages ...)

In reply to Re^2: printing hash values and keys from user (OT: stdin vs. STDIN) by AnomalousMonk
in thread printing hash values and keys from user by pinnacle

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