my $previous_line;

LINE:
while (my $line = <>) {
    next LINE if $line =~ m/disregard this line/;

    chomp $line;

    # Here's where you implement your algorithm
    # to compare the value on line $line with the
    # value on line $previous_line and print the
    # appropriate ouput line

    $previous_line = $line;
}

# Here's where you print the last line
# modified as appropriate given that
# there is no next line