use strict;
use warnings;

use Math::Combinatorics;




# Get the data file name from command line.
# Output filename is optional.

my( $infile, $outfile ) = @ARGV;

die "No filenames provided.\n" unless defined $infile;
$outfile = ( defined $outfile ) ? $outfile : $infile . '.out';




# Read input data and put it in an array for later use.

open my $in_handle, '<', $infile
    or die "Couldn't open $infile for input.\n$!";

my $dataset_aref;

while ( my $number = <$in_handle> ) {
    chomp $number;
    next unless length $number > 0;
    push @{$dataset_aref}, $number;
}

close $in_handle;





# Create a Math::Combinatorics object.

my $combinat = Math::Combinatorics->new(
    count => 2,
    data  => $dataset_aref,
);





# Open an output file, and print out the combinations.

open my $out_handle, '>', $outfile 
    or die "Couldn't open $outfile for output.\n$!";
    
while ( my @combo = $combinat->next_combination ) {
    print $out_handle "@combo\n";
}

close $out_handle
    or die "Couldn't close $outfile after output.\n$!";