use strict;
use warnings;
use Data::Dump;

my $x = 1;
my $y = 2;

print "x = $x, y = $y\n";
swap($x, $y);
print "x = $x, y = $y\n";

sub swap                                    # External SWAP?
{                                           # --------------
#   @_ = reverse @_;                        # NO
#   ( $_[0], $_[1] ) = ( $_[1], $_[0] );    # YES
#   @_[0, 1] = @_[1, 0];                    # YES - slice
#   splice @_, 0, 2, reverse @_;            # NO  - splice
#   splice @_, 0, 2,   ( $_[1], $_[0] );    # NO  - splice

#   @_[$#_ + 1] = 12;                       # Preserves aliasing
#   push @_, 'x';                           # Preserves aliasing
    unshift @_, 'y';                        # Preserves aliasing, but changes indices

    $_[1] = 55;
    dd \@_;
}