Consider:

my $value = 1;

editValue ($value);
print $value;


sub editValue {
    $_[0]++;
}
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prints '2' because @_ contains aliases to the parameters passed into the sub. If you edit the contents of elements of @_ you are editing the contents of the parameters.

You can make it more explicit by taking a reference to an element:

sub editValue {
    my $value = \$_[0];

    $$value++;
}
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