I tend to use warn (and its relatives from Carp) in production code - debugging by printing to STDERR since, as has been pointed out elsewhere on this thread, warnings are trappable - indeed, my test harnesses (using Test::More), attempt to to verify that the code is/isn't doing something unexpected by catching warnings using...

my $WARN;
local $SIG{__WARN__} = sub { $WARN = "@_" };
.
.
my $WARN = '';
some_op();
like($WARN, '/^$/', 'some_op() - no warnings');
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Update:

Added example call - for clarity.