If you think of "pass by reference" in C terms, Perl subs always receive their arguments in a "pass by reference" fashion. Arguments come in the array, @_, and the elements of that array are what we call aliases to the original values. If you modify an element of that array, you are modifying the original value.

my $so_called_constant = 'unmodified';
print "constant: $so_called_constant\n";
make_modified( $so_called_constant );
print "constant: $so_called_constant\n";
make_modified( 'literal' );

sub make_modified { $_ = 'modified' for @_ }
__END__
constant: unmodified
constant: modified
Modification of a read-only value attempted
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Note that there are other places where these aliases appear naturally, and you can create them deliberately.

What we call a "reference", a C programmer would probably call a "pointer". The brackets in "[ 1, 2, 3 ]" create a reference to an array that doesn't have a name. It's exactly like the reference you get from "\@triplet" except that there isn't a variable that holds the array whence came the reference.