Foreach loop provides list context, so foreach my $i ( @{ $aref } ) explodes into its elements

foreach my $i( ['kept', 82], ['notkept', 1], ['repaired', 3] ) { ...
# so $aref->[$i][0]
# looks like $areg->[ ['kept', 82] ][0]
#

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Here the first lines out error output: 
$ perl buggy_array.pl 2> out.txt

(it hangs here, press Ctrl-C to stop)

$ head -6 out.txt
Use of reference "ARRAY(0x1cfb148)" as array index at ary.pl line 21.
Use of uninitialized value in concatenation (.) or string at ary.pl li
+ne 21.
Use of reference "ARRAY(0x1d0fad8)" as array index at ary.pl line 21.
Use of uninitialized value in concatenation (.) or string at ary.pl li
+ne 21.
Use of reference "ARRAY(0x1d0fbf8)" as array index at ary.pl line 21.
Use of uninitialized value in concatenation (.) or string at ary.pl li
+ne 21.

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Now, why does it keep looping? 
perl -E 'my $aref=[undef]; say 0 + $aref'
19468616

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19468616 is the memory address of $aref here. Which means 
$areg->[ ['kept', 82] ][0]

# becomes something like
# $areg->[ 9999999 ][0]
# (or whatever the memory address of that array is...)

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When you do stuff like $aref->[999][0] and element 999 of $aref doesn't exist, Perl will extend $aref. It will fill it with undef, up to element 999. 
perl -E 'my $aref=[undef]; $aref->[1000][0]; say scalar @{ $aref }'
1001

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They call it 'autovivification' (sp?). (but if you do $aref->[999][500], Perl will not create the rightmost array - the one with 500 elements)

So, now your $aref has tons of of undefs. So the loop keeps looping and issuing warnings.

So yeah, all in all, that was a pretty interesting question, I must say.

In reply to Re: Dereference anonymous AoA by Anonymous Monk
in thread Dereference anonymous AoA by neilwatson

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