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my $buf; # leave value as undefined so use before a real value is avai
+lable will result in a warning message.
...;
...
    ...;
    $buf .= $newletter; # not an error because appending to an undef v
+alue is treated the same as appending to an empty string
}

    if (defined $buf) { $buf .= $newletter; }
    else { $buf = $newletter; }

my $buf = '';
...;
while (<$input>) { ...; $buf .= $newletter; }