Well, for a start, you can use 'scalar reverse $word' to reverse a word.
If you were using '&reverse_word' as simply an example, then I'd recommend reading up on shift and return;

sub reverse_word() {
    my $word = shift;
    my $reversedword = scalar reverse $word;
    return $reversedword;
}
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or, more consisely:

sub reverse_word() {
    return scalar reverse shift;   
}
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Originally posted as a Categorized Answer.