$SIG{'__WARN__'} = sub { die @_ } if $die_on_error;
warn "foooooo";
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Does not quite work if you allow your code to spit out warnings for other reasons. In that case, you have to tag your speacial warnings, e.g.:

$SIG{'__WARN__'} = sub { die @{$_[0]} if ref $_[0] eq 'ARRAY'; warn @_
+; } if $die_on_error;
warn "foooooo"; # never dies
warn ["foooooo"]; # sometimes dies
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