$. perlvar:

test.txt:

line 1
line 2
line 3
line 4
line 5
line 99
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ln.pl:

#!/usr/bin/perl

use strict;
use warnings;

open(my $fh, "<", "test.txt")
    or die "cannot open < test.txt: $!";

while (my $row = <$fh>) {
  chomp $row;
  # do something 
  print "line number $.\n";
}
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output:

line number 1
line number 2
line number 3
line number 4
line number 5
line number 6
[download]

update: forgot to add the output.