An expression is false if it stringifies to any of the following

But it wouldn't be Perl if there wasn't an exception to that:

use overload "bool" => sub {0},
             '""' => sub {"Send in the clowns"};
my $o = bless [];    # $o stringifies to "Send in the clowns"
say "Hello" if  $o;  # No stringification happening here
say "world" if "$o"; # Forced stringification
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prints just "world"