This is a well-known "trap", and can easily happen without creating such a sub. The problem is your assumption that a failed regular expression would somehow undefine $1. But it doesn't. The proper way of writing your sub is: 
    sub chg {
        $_ [0] =~ $_ [1] && $1 ? "yes\n" : "no\n";
    }

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Abigail

In reply to Re: $1 trap by Abigail-II
in thread $1 trap by chunlou

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