if the following line is different from the previous i have to print it

For that, you'd have to keep the previous line: 

my $prev;
while (<>) {
    print if defined($prev) and $_ ne $prev;
    $prev = $_;   # update $prev
}

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(The defined check would skip the first line, as there isn't really one previous to it...)

In reply to Re^3: print a determinate line number by almut
in thread print a determinate line number by Paulux

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