if (defined $p && $p ne "") { # To see if $p had non-null value
   # Do something
}

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This makes sense. It is technically redundant: 'undef' evaluates to the empty string in string context, so undef ne "" is false. However, evaluating undef in a string context gives you a warning if use warnings is turned on, so what you wrote is the proper thing to do.

In reply to Re^2: defined vs null string by matyasbot
in thread defined vs null string by Anonymous Monk

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