use strict;

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

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

sub modifyvars {

    $_[0] = 2;
    $_[1] = 3;
    $_[2] = 4;

}