The following works for me in 5.16.0:

($die_on_error ? \&CORE::die : \&CORE::warn)->($error);
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In 5.10.1, though, I am getting

Undefined subroutine &CORE::warn called
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In older versions of Perl, you can still define your own subroutines and reference them (they can even be anonymous):

($die_on_error ? sub { die shift } : sub { warn shift } )->($error);
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I do not find it more readable than

die $error if $die_on_error;
warn $error;
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