But what *is* the most important part with errors?

die "I hate you" if $string =~ m/error/i;
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or

$string =~ m/error/i and die "I hate you";
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As with most, it is a matter of taste and brain. *My* brain 100% prefers the second over the first. Note that I am not of native English tongue. That might matter.

My rule of thumb is to forbid all postfix if/unless except when the expression is a simple $debug / $verbose >= $n.