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Editorial Team
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Asked: 2026-05-22T00:50:59+00:00

when attempting to print object as in: print $response{_content} \n; printf OUTPUT $response{_content} \n;

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when attempting to print object as in:

print "$response{_content} \n";
printf OUTPUT "$response{_content} \n";

The printf statement generates error “Modification of a read-only value attempted”

It’s an intermittent error. Only happens once in a while, but this program needs to be 100% reliable. dang.

It prints fine to STDOUT.

What am I doing wrong? arrgh!

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1 Answer

  1. Editorial Team

    The first argument of printf is interpreted as output format, not output itself. See perldoc -f printf and man 3 printf for details.

    The problem is, printf might occasionally try to write to its args (this has even been the source of several vulnerabilities in C programs), for instance: 

    perl -we 'printf "abc%n\n", $_; print "$_\n";'

    As you can see, this sets $_ to 3, which is the number of characters written before %n occurred. Try %n without further args and you’ll see the exact error message from OP.

    Long story short: use print unless you really need advanced formatting. Keep first argument to printf r/o unless you really need even more advanced formatting.

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