If you neglect overloading, all values other than "0", "", (), or undef are true. Equality to zero has nothing to do with it. Consider these:

my @zeroes = qw( zero 0ButTrue 00 ); 

for my $z (@zeroes) {
    print "'$z' ", $z == 0 ? 'is ' : 'is not ', "equal to zero.\n";
    print "'$z' ", $z ? 'is ' : 'is not ', "true.\n\n";
}
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package C; 
use overload 'bool' => sub { 0 };
use overload '=='   => sub { 0 }; 
sub new { bless {} }

package main; 
my $c = C->new;

print "\$c is not equal to zero.\n" unless $c == 0; 
print "\$c is not true.\n"          unless $c;
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-sauoq
"My two cents aren't worth a dime.";

... and except for 0-but-true magic (among others).

See "What is truth? (Curiosity corner)" for more information.