My first though was along the lines of :nonegindex. But that's long and unwieldy. What about something with an argument:

# $foo[-1] returns 'Illegal Array Index Error'
@foo :negindex( value => 'Illegal Array Index Error' );

# $foo[-1] normal behavior
@foo :negindex( REVERSE );

# $foo[-1] apply some filter ( filter takes array, index as args)
@foo :negindex( filter => sub { return -1 * $_[0]->[ abs( $_[1]) ] } )
+;
 
# $foo[-1] returns undef, exists( $foo[-1] ) is false.
#@foo :negindex( DO_NOT_WANT );
@foo :negindex( NO_EXIST );

# And to anticipate all future feature creep:
# $foo[-1] returns sends an email message
@foo :negindex( email => bob@example.com );
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