What kind of variables? If just plain out scalars (number/string), then you have to pass a reference. Same for an array or hash. If you have an array ref or hash ref (or object), it's already a reference.

use Data::Dumper;
foo();
sub foo {
  my $x = 3;
  my @y = ( 1 .. 3 );
  my $yy = [ 1 .. 3 ];
  my %z = ( a => 1, b => 2 );
  my $zz = { a => 1, b => 2 };
  my $obj = My::Obj->new;  # e.g. bless'd hashref
  print Dumper [ $x, \@y, $yy, \%z, $zz, $obj ];
  bar( \$x, \@y, $yy, \%z, $zz, $obj );
  print Dumper [ $x, \@y, $yy, \%z, $zz, $obj ];
}
sub bar {
  my @args = @_;
  ${$args[0]} += 2;  # add 2 to x
  $args[1]->[1]++;   # 2->3 in y
  $args[2]->[1]++;   # 2->3 in yy
  $args[3]->{a}++;   # 1 -> 2 in z
  $args[4]->{a}++;   # 1 -> 2 in zz
}
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You can also do this:

$x = 4;
warn $x;
blah($x);
warn $x;
sub blah {
  $_[0] += 3;
}
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