use strict;
use warnings;

use Data::Dumper;

# the first arg will modified
sub square_first_arg {
    $_[0] = $_[0]** 2;
} 

my $x = 2;
square_first_arg($x);
print "2 squared is $x\n";

eval {
  square_first_arg( 3 );
};

print "trying to square something unmodifyable: $@ \n";

# Referenzen

my $a = 'A';

sub a_to_b {
    # copy @_, as we dont't want to modify the the param itself
    my ($first_param) = @_;

    # modify the scalar that $first_param refers to
    ${ $first_param } = 'B';
}

print Dumper( $a );
a_to_b( \$a );
print Dumper( $a );