$_ iterates through that list (but why wasn't my original say for iterating through the list?)

@ dereferences the array (again, why didn't my original @{...} dereference it?)

It was iterating through the list and it did dereference , but the number 2 is not an array reference, so the list was empty :) strict noticed this problem, so it ended your program . This is why we love strict.

Whatever text you're learning from, you need to go back and do the exercises you skipped :) the only way to absorb and remember is by typing it yourself :)

$ perl -E " say  for [1..4],[5..8] "
ARRAY(0x3f8adc)
ARRAY(0x9b9eac)
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aww, array references stringify like that, to get at the values, we need to dereference

Trying it your way

$ perl -E " say  for @{ [1..4],[5..8] }"
5
6
7
8
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Aww, it only printed the contents of the last reference. To print both, we need to iterate over a list of reference, not a list of values

$ perl -E " say @$_ for [1..4],[5..8] "
1234
5678
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Oh look, the numbers are all joined together, thats how say works

$ perl -E " say 1,2,3,4; say 5..8; "
1234
5678
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I know, we'll put it in double quotes string, that will improve it

$ perl -E " say qq{@$_} for [1..4],[5..8] "
1 2 3 4
5 6 7 8
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Now for the brackets

$ perl -E " say qq{[@$_]} for [1..4],[5..8] "
[1 2 3 4]
[5 6 7 8]
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Now for the comma by changing the $", the LIST_SEPARATOR (the \" is for my shell, cmd.exe )

$ perl -E " local $\"=q{,}; say qq{[@$_]} for [1..4],[5..8] "
[1,2,3,4]
[5,6,7,8]
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Even if we employ $" properly by limiting the scope of the changes with local, its better to use join function

$ perl -E " say q{[}, join( q{,},  @$_ ), q{]} for [1..4],[5..8] "
[1,2,3,4]
[5,6,7,8]
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The values function is context sensitive

$ perl -E " %f = 6..9; say for values %f; "
9
7

$ perl -E " %f = 6..9; say for scalar values %f; "
2
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Using @{} puts values %f in scalar context, just like the scalar function. This makes the values function return the number of values in %f; The number of values is not an array reference.

To learn more about context see Tutorials: Context in Perl: Context tutorial, or see the free book Modern Perl, a loose description of how experienced and effective Perl 5 programmers work....You can learn this too.