I would use a hash, like this:

if (/$word/i) {
    print unless (++$matches{$word} > 1)    
}
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I suppose you could use $_ in place of $word if you wanted.

Update: After seeing swiftone's post I realize that I forgot to adjust case. Thanks swiftone.

if (/$word/i) {
    print unless (++$matches{lc $word} > 1)    
}
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