0 .. $#categories runs through 0, 1, & 2; but the third element of your @data only contains 2 elements: [7,8], so when you do my $val = $d->[$c]; on the third iteration of the outer loop, $val gets set to undef hence the error when you try to use $val as a hash key in line 23.

The solution is "don't do that" :)

if( defined $val ) {
        $h{$val}=$cat;
}
else {
     ## Do something else (or nothing).
}
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What alternative action you should take is very much dependant upon your application requirements.