This can't be done with simple ties.

It can be done with Test::Trap, though. :) (Shameless as ever ....)

use Test::Trap qw/ :flow:stderr(systemsafe) /; # or similar
trap { system 'command', 'with', 'args' };
my @logmessages = split /\n/, $trap->stderr; # chomped! or season to t
+aste
[download]

Not exclusively for use in test scripts, indeed. :)