The difference lies in the fact that Perl automatically flattens all lists in a comma separated expression:

print $_ for @arr1, @arr2

is (for this example) the same as

my @temp = @arr1;
push @temp,@arr2;
print $_ for @temp;
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where your second example does not have implicit list flattening as you are dealing with references. Your second example

print @$_ for ( [ 'a' .. 'e' ], [ 0 .. 4 ] );
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is equivalent to

print @$_ for \@arr1, \@arr2
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and to get the equivalent to your first example, you have to turn the references back into the list elements first, for example by using map, which can return multiple output elements for one element :

print $_ for (map { @$_ } \@arr1, \@arr2);
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