DB<108> sub inc_a { $_[0]++; return } DB<109> $x=1; inc_a($x); print $x 2 DB<110> inc_a(1) Modification of a read-only value attempted at (eval 13)[/usr/share/perl/5.10/perl5db.pl:638] line 2. #### DB<111> sub inc_b { my $r=\ $_[0]; $$r++; return } DB<112> $x=1; inc_b($x); print $x 2 DB<113> inc_b(1) DB<114>