The problem is in the backticks. You'll need to untaint your word with regular expression match. One way to do it:

    my $_word = defined($_=shift) ? $_ : die "usage: $0 <word>\n";
    chomp($_word);
    my ($word) = $_word =~ /([[:alnum:]]+)/;
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Perl thinks that $word might be dangerous, so it's trying to prevent you from performing what it thinks is an unsafe operation. See perlsec for details.