Silencing STDERR and warnings

Question

How does warn 4 differ from print STDERR 4?

perl -e 'local *STDERR; warn 4'

(output still goes to STDERR)

perl -e 'local *STDERR; print STDERR 4'

(no output here)

Answer 1

• warn triggers $SIG{__WARN__}.
• warn doesn't use $\ or $,.
• And warn apparently uses the file handle in the original STDERR, as you've demonstrated^[1].

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1. Not quite. Your code could also demonstrate that warn uses fd 2 directly, but that's disproven by

   close(STDOUT);
   close(STDERR);
   open(STDERR, '>file');
   warn(fileno(STDERR)."abc");  # 1abc
   

Answer 2

You haven't silenced the STDERR handle yet. In order to really silence it, you need to say:

perl -e 'local *STDERR; open(STDERR, ">/dev/null") or die $!; warn 4'

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perldoc perlvar tells:

As the 'IGNORE' hook is not supported by __WARN__, you can disable warnings using the empty subroutine:

 local $SIG{__WARN__} = sub {};