In foreach, the loop variable (default $_) is an alias to the original value. That means that changing the value will change the original.

my $x = 100;
my @y = (10, 20, 30);
foreach ($x, @y) {
    $_ *= 2;
}
local $" = ", ";
print "\$x = $x\n\@y = (@y)\n";
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# or:
foreach my $n ($x, @y) {
    $n *= 2;
}
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Ditto in the block or expression for map and grep ($_ only).

map { $_ *= 2 } $x, @y;
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grep { $_ *= 2 } $x, @y;
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BTW parameters to subs are aliases too, so if you change them in @_, you'll modify the originals too:

sub double {
    foreach(@_) {
        $_ *= 2;
    }
}

double($x, @y);
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