issues for a code snippet to handle the input file - perl

Question

I am studying a Perl program, which includes the following segment for handling an input file. I do not understand what is s/^\s+//; used for? Moreover, what are '|' and '||' stand for in open(FILE, "cat $fileName |") || die "could not open file";
open(FILE, "cat $fileName |") || die "could not open file";
while (<FILE>)
{
s/^\s+//;
my #line = split;
if ($line[0]!~ /\:/) {$mark=0}
my $var = $line[$mark];
## some other code
}

Answer 1

You can read the documentation for the various functions in perlfunc.
This code will open a file for reading, by the rather circumspect way of piping from cat instead of simply opening the file. The | means that the shell command cat is piped to the open command, and our file handle will read from the output.
|| is simply or. Open the pipe, and if that fails, the program dies.
while(<FILE>) will read through every line of the input and assign each line to $_. That line is then used implicitly in the substitution and split below. I.e. s/^\s+// is equal to $_ =~ s/^\s+//, and split is equal to split(' ', $_).
s/^\s+//
Will remove leading whitespace. The split will split each line on whitespace, and the elements are stored in the array #line.
Because of the use of implicit split on whitespace, the stripping the leading whitespace with s/^\s+// is not really needed, as that is done automatically.
If the first element does not contain a colon :, $mark is set to 0. Otherwise, it is not set, and will presumably use the value from the previous iteration, since it is not defined inside the loop. Finally, $var is initialized as element number $mark, which is either 0 or whatever.
ETA: As a rather insidious oops: If $mark is undefined, i.e. it does not contain a colon, then $var will still be assigned $line[0], since undef will be converted to 0, with a warning. If use warnings is not in effect, this error is silent, and therefore insidious.
This code seems to be written by someone who does not know too much about perl, and it might not be very safe to use.

Answer 2

The substitution trims leading whitespace that appears at the beginning of the line (^), leaving any non-whitespace characters as the first.
The || operator in open... || die ... is a high-precedence or. If open fails, die executes.
open(FILE, "cat $fileName |") is a waste of an external process. To read a file for input, simply do:
open FILE, '<', $filename or die qq{Could not open "$filename" for reading: $!};
The parentheses for the open call are optional because or does not bind tightly.
It is also better to use lexical file handles:
open my $fh, '<' $filename or die qq{Could not open "$filename" for reading: $!};
This file handle is assigned to a lexical variable that lives only within the scope it is declared. Once the program flow exits this scope, the file closes automatically.

Answer 3

Part of the confusion is that the developer is using the default variable, $_. Many Perl commands (I would say about 1/3 of them) act upon $_ when you don't specify the name of the variable in the function. For example, these are syntactically the same:
my $uppercase_name = uc($_);
my $uppercase_name = uc;
In both cases, the uc function will print the string in the $_ variable in upper case characters. In fact, even the print statement uses the $_ variable. Again, these are both the same:
print $_;
print;
It's frowned upon to use the default variable in newer Perl scripts because it doesn't add clarity to the program and it doesn't make the program faster. I've rewritten the same code snippet you used in order to show the missing $_ variable. It might make the code easier to understand:
open(FILE, "cat $fileName |") || die "could not open file";
while ($_ = <FILE>)
{
$_ =~ s/^\s+//;
my #line = split $_;
if ($line[0] !~ /\:/) {
$mark = 0;
}
my $var = $line[$mark];
## some other code
}
Notice that the while statement is putting the value of the line read into the $_ variable and that the substitute command (the s/^\s+//) is also operating on the $_ variable. I hope that clarifies the code a bit for you.
Now for your questions:
_[W]hat do '|' and '||' stand for?
The || means or as in do this or that. In practice, the or can be thought of as an if statement:
if (not open(FILE, "cat $fileName |")) {
die "could not open file";
}
That is, if the open statement failed, then execute the die statement. If the open statement did manage to open the file, then don't execute the die statement.
In Perl, you now see or instead of || in cases like this:
open(FILE, "cat $fileName |") or die "could not open file";
which makes the meaning a bit more obvious: Open the file, or kill the program.
The single pipe (|) at the end of the file name means execute the command in the open statement (the cat $filename) and read from the output of this command. Imagine something like this:
open (COMMAND, "java -jar foo.war|") or die "Can't execute 'java -jar foo.war'";
Now, I'm running the command java -jar foo.war and using its output in my Perl script.
You can do this the other way around too:
open (MAIL, "|mail $recipient") or die "Can't mail $recipient";
print MAIL "Dear $recipient\n\n";
print MAIL "I hope everything is well.\n";
print MAIL "Sincerely,\n\nDavid";
close MAIL;
I'm now opening the command mail $recipient and writing to it with the print statements. In this case, I'm emailing $recipient with a simple message.
I do not understand what is s/^\s+//; used for?
In the original program, it was on a line by itself:
s/^\s+//;
I've added the missing variable which should help clarify it a bit:
$_ =~ s/^\s+//;
This is the Substitute command in Perl. It's taking the $_ variable and substituting the regular expression ^\s+ with nothing. If you don't understand what are regular expressions, you should take a look at the Perldoc tutorial on the subject. Basically, this is removing all spaces, tabs, and other forms of white space from the beginning of the line.