Just put a backslash in front of the dollar-sign when you pass the variable to the function.

Don't forget to de-reference the variable inside the function ...

use strict;
use warnings;

my $x = 10;
&function( \$x ); # if I change $x = 5
print $x; # $x = 10

sub function {
    my $x = $_[0];

    $$x = 5;
} # End function subroutine
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