Yeah, printing to a read-only filehandle isn't going to work :-)

How about redirecting the second program's STDIN so that it reads from your data. Something like this (untested).

# Read all the data from STDIN in the $buffer variable
my $buffer = do { local $/; <STDIN> };

#Do a subtitution.
$buffer =~s/essage/assage/sg;

system("echo $buffer | perl printout.pl");
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Update: Or what Ovid said. Much better idea.