Get The Last Modification Time Of A File 
#
## get the modification time of a file
#
    $mtime = (stat($file))[9];


 Get the Size of a File 
#
## get the size of a file
#
    $size = (-s $file);


 read input interactively 
#
## read input line into variable $buf
#
    print "Enter a number : ";
    chomp($buf = <STDIN>);

 Read a File quickly into a variable 
#
## slurp a file -- read $size number of bytes into variable $buf
#
    $size = (-s $file);
    open(IN,"$file");
    read(IN,$buf,$size);
    close(IN);


 Creating Directories 
#
## To avoid umask calculations, 
## save the current umask while setting the umask to 000, 
## make the directory, then set the umask back to what it had been
#
    if (! -d $localdir)
    {
        $old = umask(000);
        mkdir($localdir,0775); 
        umask($old);
    }


 Get The Current Directory 
#
## get the current directory
#
    use Cwd;
    $pwd = cwd();


 Read A Directory 
#
## read a directory $dir ( ignoring . and .. )
#
    opendir(DIR,"$dir") || die "opendir $dir failed";
    @files = grep(!m#^\.\.?$#,readdir(DIR));
    closedir(DIR);


 Recursively Read a Directory 
#
## recursively read a directory, $startdir finding all files
## at the end all directory names are in @dirs, all files are in @files
#
    push(@dirs,$startdir); 	# add an element to an array
    foreach $dir (@dirs)	# loop through the elements of an array
    {
        opendir(DIR,"$dir") || die "opendir $dir failed";
        @list = grep(!/^\.\.?$/,readdir(DIR));
        closedir(DIR);
    
        foreach $item (@list)
        {
            $fullname = $dir."/".$item;	# add the directory name to the file name
            if (-d $fullname)
            {
                push(@dirs,$fullname);
            }
            elsif (-f $fullname)
            {
                push(@files,$fullname);
            }
            else
            {
                print "What?: $fullname\n";
            }
        }
    }


 Breaking out or "short circuiting" a loop 
#
## break out of a loop
#
    open(PS,"/bin/ps -ef |");
    while ($procline = <PS>)
    {
        if ($procline =~ m#httpd#)	# find a webserver
        {
            ($junk,$user,$pid,$ppid) = split (/\s+/,$procline);	# split the line on whitespace
            last;     # jump out of the while loop
        }
    }
    close(PS);


 Determining The PID of a Child Process 
#
## determining the PID of a child process
#
    $childpid = open(PS,"/bin/ps -ef |");


 get the loginID 
#
## get the loginID 
#
    $user = (getpwuid($<))[0];


 get the hostname 
#
## get the hostname 
#
    use Sys::Hostname;
    $host = hostname();


 concatenate the elements of an array, separated by $delimiter 
#
## concatenate the elements of an array, separated by $delimiter
#
    $list = join($delimiter,@groups);

 Working with references to a hash 
#
## concatenate the elements of an array, separated by $delimiter
#
my (%mainhash);

# pass the reference to mainhash by putting the "\" (slash) in front
update(\%mainhash);
printit(\%mainhash);

sub update
{
    ($hash) = @_;
    #
    # #update the "referenced" hash by using $$
    #
    $$hash{"a"} = 1;
    $$hash{"b"} = 2;
    $$hash{"c"} = 3;
    $$hash{"d"} = 4;
}

sub printit
{
    ($hash) = @_;
    #
    ## "keys" cannot work with the reference $$hash so use %{$hash}
    #
    foreach $key (keys %{$hash})
    {
        print "$key\n";
    }
}




 print using a "here" document 
#
## print using a "here" document -- print out html code without lots of quoting games
## variables between  <<End_Text; and  End_Text  will be expanded
## note that End_Text MUST start in the first column
#
    print <<End_Text;
  <SCRIPT LANGUAGE=JAVASCRIPT>
  <!--
  document.write('<img src="/cgi-bin/get-img.cgi?'+document.referrer+'"alt="logo" height=103 width=419>')
  // -->
  </script>
  <NOSCRIPT>
  <img src="/cgi-bin/get-img.cgi?noscript" alt="logo" height=103 width=419>
  </NOSCRIPT>

End_Text


  
 parse script arguments -- either from the command line or as a cgi script 

#
## parse script arguments -- either from the command line or as a cgi script
## Note: Arguments are passed to a cgi script in an environment variable 
##       named QUERY_STRING.  These arguments are usually in the form of 
##       name=value pairs separated by an ampersand (&)
##       CGI:          test.pl?one=1&two=2
##       Command Line: test.pl one=1 two=2
## If the QUERY_STRING variable does not exist, then we can assume the script 
##  is running from the command line (it might be running as a CGI with no 
##  inputs, but then there will be no command line inputs either)
## If the script is running from the command line, add the command line arguments
##  to the QUERY_STRING environment variable.
## This simplifies argument handling -- the same routine can then be used to parse 
##  the arguments whether they come form the command line or from a web server.
## This routine stores the name-value pairs in a hash named inputs
##
## THIS DOES NOT CHECK FOR BAD OR DANGEROUS CHARACTERS SUCH AS SHELL ESCAPES!!!!!
#

# command line inputs
    if ($ENV{QUERY_STRING} eq "")
    {
        $ENV{QUERY_STRING}   = join("&",@ARGV);
        $ENV{REQUEST_METHOD} = "GET";
    # set a "default" value
        $inputs{file}        = "../data.htm";
    }
    
# cgi-script

    if ($ENV{REQUEST_METHOD} eq 'GET')
    {
        # split the query into keywords
        foreach $input (split("&",$ENV{QUERY_STRING}))
        { 
            if ($input =~ /(.*)=(.*)/)
            {
                ($key,$value) = ($1, $2);
                if ($debug) { print "[$key] [$value]<br>\n"; }
                # replace "+" with " "
                $value =~ s/\+/ /g ;                          
                # convert hex characters
                $value =~ s/%(..)/pack('c',hex($1))/eg;
                # add keyword/value pair to a list
                $inputs{$key} = $value; 
            }
        }
    }