The example you linked to deals with files with fixed-length records - i.e. generally binary files. As you're talking about "line by line", I assume you're talking about a text-based file - e.g. plain text, HTML, CSV, etc.

# Set the character which will be used to indicate the end of a line.
# This defaults to the system's end of line character, but it doesn't
# hurt to set it explicitly, just in case some other part of your code
# has altered it from the default.
local $/ = "\n";

# Open the file for read access:
open my $filehandle, '<', 'myfile.txt';

my $line_number = 0;

# Loop through each line:
while (defined($line = <$filehandle>))
{
  # The text of the line, including the linebreak
  # is now in the variable $line.

  # Keep track of line numbers
  $line_number++;

  # Strip the linebreak character at the end.
  chomp $line;

  # Do something with the line.
  do_something($line);

  # Perhaps bail out of the loop
  if ($line =~ m/^ERROR/)
  {
    warn "Error on line $line_number - skipping rest of file";
    last;
  }
}
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But we can make the above more concise, because Perl usefully defines a variable called $_ which is used as a default variable in many cases; and a variable called $. which keeps track of the current line number.

# Set the character which will be used to indicate the end of a line.
local $/ = "\n";

# Open the file for read access:
open my $filehandle, '<', 'myfile.txt';

# Loop through each line:
while (<$filehandle>)
{
  # The text of the line, including the linebreak
  # is now in the variable $_.

  # Strip the linebreak character at the end.
  chomp;

  # Do something with the line.
  do_something($_);

  # Perhaps bail out of the loop
  if (m/^ERROR/)
  {
    warn "Error on line $. - skipping rest of file";
    last;
  }
}
[download]