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

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

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

print Dumper \%hash;
__END__
[ 4:49:09.65] C:\test>junk.pl
syntax error at C:\test\junk.pl line 7, near "} @"
Execution of C:\test\junk.pl aborted due to compilation errors.