use strict;
use warnings;
open(my $fh, '<', "$file") or die $!;#use three argument open
my $previous_line = q{}; #initially previous line would be empty
while(my $current_line = <$fh>){
  chomp;
  print"$current_line\n";
  print"$previous_line\n";
#assign current line into previous line before it go to next line
  $previous_line = $current_line; 
}
close($fh);