#! perl -slw
use strict;
use Data::Dumper;

my $d = [ [[1,2], [3,4]], [[5,6], [7,8]] ];

my %hash = map{
    my $key = "@{ $_->[ 0 ] }";
    $key => $_
} @{ $d };

print Dumper \%hash;

__END__
C:\test>junk.pl
$VAR1 = {
          '5 6' => [
                     [
                       5,
                       6
                     ],
                     [
                       7,
                       8
                     ]
                   ],
          '1 2' => [
                     [
                       1,
                       2
                     ],
                     [
                       3,
                       4
                     ]
                   ]
        };