I prefer making copies as they are needed, rather than all at once:

for (@a) {
    my $b = $_; # Or do a deep copy, if necessary
    $b *= 2;
}
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For making a temporary array, I like this idiom, though there's no performance reason to prefer it to any other:

for my $b (map $_, @a) {
    $b *= 2;
}
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others prefer

for my $b (@{[@a]}) {
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