use strict;

my $X = 1;
my $Y = 2;
my $Z = 3;

print "before: X=$X, Y=$Y, Z=$Z\n";
pass_by_ref(\$X, \$Y, \$Z);
print "after: X=$X, Y=$Y, Z=$Z\n";

sub pass_by_ref
{
    my ($x, $y, $z) = @_;
    
    ${$x} = 2;   # or $$x = 2;
    ${$y} = 3;
    ${$z} = 4;
}