The warning message is printed to stdout instead of stderr, meaning it gets mixed in with output from other tools which may be intended to be machine-parsed.

The specific example I have in mind is the xsubs deparser in IntelliJ IDEA, which outputs an invalid .pm file beginning as follows:

Warning: This script should be run inside irssi
#
#
# DO NOT EDIT THIS FILE, IT'S AUTOGENERATED FROM XS MODULES AND WILL BE UPDATED AUTOMATICALLY
#
#
package Async::Interrupt {
#... etc