here a little demo using eval

NB: the allowed vars must be bound inside the setter sub, otherwise the eval will fail.

use v5.12;
use warnings;

{

    package TEST;

    my $param = 666;

    sub getset {
        my %args = @_;
        return [$param]                 # list of allowed vars
            unless @_;
        while ( my ( $var, $val ) = each %args ) {
            eval "$var = $val";
        }
    }

    sub show {
        say $param;
    }

}

package main;

TEST::show();
TEST::getset( '$param' => 42 );
TEST::show();
[download]

666
42
[download]