Something similar to this should do it. The trick is not to update $lastline until you don't have a match. As a side note, in the past I have successfully loaded around 100K lines into an array. You may be surprised at what Perl can handle.

open (IN, "$input_file");
open (OUT, ">$output_file");

my $lastline = <IN>;
print OUT $lastline;
while(<IN>){
    my $line = $_;
    if ($line eq $lastline){
        next;
    }
    print OUT $line;
    $lastline = $line;
}

[download]

update: corrected typo

In reply to Re: Read file line by line and check equal lines by hangon
in thread Read file line by line and check equal lines by Anonymous Monk

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