Let's say foo.txt contains 5 lines of text.

Line one
Line two
Line three
Line four
Line five

$. is the default Perl variable that captures the number of lines in a file.

open(FH, "<", "foo.txt");

while (my $line = <FH>) {
  print "$. $line";
}

close FH;

The prior markup used $. to print each line number and $line to print each line, which would produce the following output.

1 Line one
2 Line two
3 Line three
4 Line four
5 Line five

This will count the total number of lines in the file.

open(FH, "<", "foo.txt");

my $count = 0;

while (<FH>) {
  $count = $.;
}

close FH;

print "$count\n";

To count the number of lines in a file that contain the text "Hello World".

my $file = "/path/to/file.txt";
open(FH, "<", $file) or die "cannot open $file $! \n";

my $count = 0;

while (<FH>) {
  $count++ if /Hello World/;
}

close FH;

print "$count\n";