sub print_local_var_plus_one {
    my ($var) = @_;
    print ++$var;
}
sub print_var_plus_one {
    print ++$_[0];
}

my $x = 2;
print_local_var_plus_one($x); # prints "3", $x is still 2
print_var_plus_one($x);       # prints "3", $x is now 3 !
print $x;                     # prints "3"