This is documented in perlsub:

Similarly, in the conditional

    if ((my $answer = <STDIN>) =~ /^yes$/i) {
        user_agrees();
    } elsif ($answer =~ /^no$/i) {
        user_disagrees();
    } else {
        chomp $answer;
        die "'$answer' is neither 'yes' nor 'no'";
    }
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the scope of $answer extends from its declaration through the rest of that conditional, including any elsif and else clauses, but not beyond it.