As to return by value, an example:

#!/usr/bin/perl
use 5.016;
use warnings;
use Data::Dumper;

# pass by ref; return by val

my $foo = "abc";

sub subr {
  my $local_foo = shift;            # see Note1, below
  my $local_deref = $$local_foo;
  say " In sub, \$local_deref: $local_deref";
  $local_deref .= " def";
  say "\t  And now, after cat, \$local_deref: $local_deref";
  return $local_deref;
}

my $fooref = \$foo;

my $result = subr($fooref);

say "\t\t Back in main, after subr has returned.";
say "\t\t $result";
[download]

Produces output:

 In sub, $local_deref: abc
          And now, after cat, $local_deref: abc def
                 Back in main, after subr has returned.
                 abc def
[download]

Note1: Had you attempted to -- for example --  print "DEBUG: \$local_foo: $local_foo"; before the deref you would be advised:

 DEBUG: $local_foo: SCALAR(0x169ef64)

... where the hex value (address) will vary with machine, OS or, possibly, whether the butterfly's wings are flapping upward or downward in the jungles or Borneo.