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my @foo = (2, 4, 8);
foreach my $num (@foo) {
    $num *= 2;
}
$, = ",";print @foo;print "\n";

my @foo = (2, 4, 8);
foreach my $num (@foo) {
...
    $_[0] =  2*$_[0];  
    return $_[0];
}

sub double {
    my $num = $_[0];
    $num =  2*$num;  
    return $num;
}