The same thing tobyink and tye said, though more obtusely and snarkily.

Here's another example:

tie my $var, 'Some::BlackBox::Module';

$var = "45";

if ( $var ) { print "var is true\n" }  # doesn't print! WTF?
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The mere fact that an expression evaluates to a true value in scalar context doesn't mean it is true.

tie and overload are among the user-accessible features which can muddy the true/false waters.

An example like this should make the principle more obvious:

my $obj = ObjectWithLotsOfOpaqueStructure::AndLotsOfHiddenState->new(
   -use_overloading => 1,
);

if ( $obj ) {
   print "obj is 'true'.\n";
   # What does this even mean?
   # Maybe obj is an iterator with another available item?
}
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