Running this on GNU/Linux, perl v5.8.5

printing just what goes to STDERR

05:33 PM george@gnistorica george $ perl 435258.pl 1> /dev/null
#1: warning at 435258.pl line 9.
#1: STDERR is open
#1: warning at 435258.pl line 12.
#2: warning at 435258.pl line 17.
#2: STDERR is open
#2: warning at 435258.pl line 20.
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printing just what goes to STDOUT

05:33 PM george@gnistorica george $ perl 435258.pl 2> /dev/null
#1: STDOUT is open
#4: warning at 435258.pl line 32.
#4: STDOUT is open
#4: warning at 435258.pl line 35.
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So i assume that as soon you have opened STDOUT, warn uses it to spell out warnings.
otherwise, print STDERR ... does nothing.