Sorry, but that's not true. It works.

use strict;
use warnings;
use Data::Dumper;
use Tk;

my $t=MainWindow->new();
my $c=$t->Scrolled('Canvas')->pack;
$c->createArc(5,5,100,100);
$t->update;

my @a1 = @{ $c->bbox('all') || [ 1,2,3 ] };
print(Dumper(\@a1));          # 51,3,102,55

my @a2 = @{ $c->bbox('FOO') || [ 1,2,3 ] };
print(Dumper(\@a2));          # 1,2,3
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Because it returns undef (not []) on error as I was assuming.

my $rv = $c->bbox('FOO');
print(Dumper($rv));           # undef
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My mistake... sorry. I tested it directly with []...