my $var = 'foo';

warn $var; # prints foo
by_val( $var );
warn $var; # prints foo
by_ref( \$var );
warn $var; # prints bar

sub by_ref{
  my $var_ref = shift;
  $$var_ref = 'bar';
}

sub by_val{
  my $var = shift;
  $var = 'bar';
}