1 #!/usr/bin/perl
  2 use strict;
  3 use warnings;
  4 
  5 open my $FH , "+>", "input";
  6 
  7 my @contents = <$FH>;
  8 print "@contents\n";
  9 
 10 print $FH "4\n";
 11 
 12 @contents = <$FH>;
 13 print "@contents\n";
 14 close($FH);

Before executing the perl script: Initial file
ananda@ananda-desktop:~/Documents/perl_programs/try$ cat input 
1
2
3
ananda@ananda-desktop:~/Documents/perl_programs/try$ perl test.pl 


ananda@ananda-desktop:~/Documents/perl_programs/try$ cat input 
4
ananda@ananda-desktop:~/Documents/perl_programs/try$ 

After executing the perl script: Updated file