Not doing a very good job of reading the OP, people.

use strict;
use warnings;
my ($a);
my @b=(2,4,6,7);
foreach $a (@b) { print $a}
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That works. What if I want to use one element of an array as the iterator? After all, $a[1] is really just a scalar, right?

use strict;
use warnings;
my @a=(1,2,3);
my @b=(2,4,6,7);
foreach $a[1] (@b) { print $a[1] }
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Syntax error though, "near $a[". My guess is perl expects a scalar variable here, and $a[1] isn't a valid scalar variable name, even though it represents a scalar value.