#!/usr/bin/perl  

use strict;
use warnings;

my %file1_data;
my %file2_data;
 
print "\n\n"; 

# Process data files
ProcessFile( "File1", \%file1_data );
ProcessFile( "File2", \%file2_data );

# Format output
my @keys = sort keys %file1_data;
foreach my $key ( @keys ) {
    
    my $target = exists $file2_data{$key} ?
                 $file2_data{$key}        :
                 '';

    print "$key  $file1_data{$key}  $target\n";

}


# Process a file write data into supplied hash ref.
sub ProcessFile {
    my $filename = shift;
    my $data     = shift;  # Pass data by reference - big hashes used here.

    open( DATAFILE, '<', $filename ) 
        or die "Unable to open $filename - $!\n";

    # Store data in hash.  
    # Only the last instance of any key is stored.
    while ( my $line = <DATAFILE> ) {
        my ($key, $value) = split /\s+/, $line, 2;  # split into max of 2 fields.

        $data->{$key} = $value;

    }

    close( DATAFILE ) 
        or die "Unable to close $filename - $!\n";

}