A:50:r1
B:45:r2
C:40:r3

##</code><code>##

##########################################
# FINDING DETAILS OF A STUDENT:      #####
##########################################
#!/usr/bin/perl
use warnings;
use strict;   
########################
# DATABASE Input:    ###
######################## 
print "\n\n Please ENTER Database Name (.txt): ";
 my $DNAfilename=<STDIN>;
 chomp $DNAfilename;   
# open the file, or exit:  
 unless (open(DNAFILE, $DNAfilename) ) {
 print "Cannot open file \"$DNAfilename\"\n\n";
 exit;}    
 my @DNA= <DNAFILE>; 
print"\n $DNAfilename Database:\n\n";
print @DNA;
print"\n";
##########################
## CODE STARTS HERE:  
##########################
 {package Student;
   sub new     
   {my ($name, $age, $regd_no)=@_;
        my $self={ NAME => $name, AGE => $age,
                   REGD_NO => $regd_no};
        return bless $self, my $class;print"\n class: $class\n"; }

    sub display
    { my ($self) = @_;
      if ($self)
        { print 'Student Name:  ', $self->{NAME},      "\n";
            print 'Age (yr):    ', $self->{AGE},       "\n";
            print 'Regd no:     ', $self->{REGD_NO},   "\n";
            }
    }
 }    
my @students;
while (<DNAFILE>) {chomp; push @students, Student->new(split /:/);} 
print"\n WELCOME to Students' Database:\n"; 
print"\n Enter the Registration Number to view the details: ";
 my $reg=<STDIN>; chomp $reg; 
 find_student($reg)->display(); # LINE 45 
 sub find_student
  {my ($regd_no) = @_;
  print"\n\n";
  print"\n The details of the student are:\n\n"; 
  for (@students) {return  $_ if $_->{REGD_NO} eq $regd_no;}
  return undef;}
  print"\n\n";

##</code><code>##

Microsoft Windows [Version 6.1.7600]
Copyright (c) 2009 Microsoft Corporation.  All rights reserved.
C:\Users\x>cd desktop
C:\Users\x\Desktop>up1.pl


 Please ENTER Database Name (.txt): rc.txt

 rc.txt Database:

A:50:r1
B:45:r2
C:40:r3

 WELCOME to Students' Database:

 Enter the Registration Number to view the details: r1

 The details of the student are:

Can't call method "display" on an undefined value at C:\Users\x\Desktop
\up1.pl line 45, <STDIN> line 2.

C:\Users\x\Desktop>