use strict;
use warnings;

my @data=(
    [1,2,3],
    [4,5,6],
    [7,8],
    [10,11,12]
);


my @categories=qw/a b c/;
my @master;

foreach my $d(@data){
    my %h;
    foreach my $c(0 .. $#categories){
        my $val=$d->[$c];
        my $cat=$categories[$c];

        $h{$val}=$cat;
    }

    push @master, \%h;
}


##</code><code>##

Use of uninitialized value $val in hash element at ./test.pl line 23.