There's a couple of options. You can use the aliasing behavior of @_, or you can pass an actual reference.

sub func_alias {
  $_[0] = 5;
}

sub func_ref {
  my $ref = shift;
  $$ref = 5;
}

my $x;
func_alias($x);
print "x = $x\n";

my $y;
func_ref(\$y);
print "y = $y\n";
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The latter is probably better, as it is less likely to cause surprise the caller (ie, it's more explicit that you're potentially modifying the function parameter).