... Also you can write

unless ($prev eq $line){
    push (@filtered,$prev);                
}
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I would go a little further by using a statement modifier and I'd also omit the brackets around the push arguments. That saves typing two pairs of brackets and a pair of braces and, to my eye, looks clearer; others may disagree.

push @filtered, $prev unless $prev eq $line;
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Cheers,

JohnGG