#!/usr/bin/env perl
use strict;
use warnings;

my %students;
my $key = shift || 0;

while (<DATA>) {
    chomp;
    my ( $name, $age, $reg_no, $phone, $marks, $year ) = split ":";

    $students{$reg_no} = { name  => $name,
                           age   => $age,
                           phone => $phone,
                           marks => $marks,
                           year  => $year,
                         };
}

if ($key) {
    # Print the details of our preselected student
    print "Information for student with registration number $key is: \n";
    print " Name: " . $students{$key}{name} . "\n";
    print " Age: " . $students{$key}{age} . "\n";
    print " Phone: " . $students{$key}{phone} . "\n";
    print " Marks: " . $students{$key}{marks} . "\n";
    print " Year: " . $students{$key}{year} . "\n";
}
else {
    # Print the name for each student we have a record for
    print "List of all students:\n";
    foreach my $id ( sort keys %students ) {
        print " Registration Number: " . $id . "\n";
        print "   Name: " . $students{$id}{name} . "\n";
    }
}

__DATA__
John Doe:20:x1:555-1234:50:2013
Jane Doe:19:x2:555-4321:55:2012
Bob Smith:22:x3:555-2468:90:2011
Mortimer Jones:18:x4:555-0313:77:2008