use IO::Prompt::Tiny 'prompt';
use List::Util       'max';
use File::Slurp      'read_file';

my @files =
  map {
    my $fn = $ARGV[$_] // prompt 'Please enter filename #' . ( $_+1 ) . ':';
    read_file $fn, array_ref => 1, chomp => 1;
  } 0 .. 1;

for( 0 .. max map { scalar @{$_} } @files ){
  my $left_line  = $files[0][$_] // '';
  my $right_line = $files[1][$_] // '';
  print "$_: $left_line\t$right_line\n";
}