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my $a = 10;

...
sub change {
    $_[0]++;
}

my $a = 10;

...
    my $param = shift;
    $param++;
}

# contrived example: there usually isn't a good reason
# to have references to scalars.
...
    my $param = shift;
    ${$param}++;
}

my $a = 10;
my $b = \$a;
...
sub change {
    $_[0] = \$c;
}