If you pass an array or a hash, then modifying it should not modify the original.

use YAML qw( Dump );

sub yarly { $_[0] = "O RLY?" }
my @a = ( "b", "c", "d" );
print Dump( \@a ), "\n";
yarly( @a );
print Dump( \@a ), "\n";

__END__

--- #YAML:1.0
- b
- c
- d

--- #YAML:1.0
- O RLY?
- c
- d
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Elements of @_ are aliases for the originals.