% perl -wle 'sub foo { 1 for @_ } print 2 if defined foo()'
foo does nothing. There is no parameter. So undef is returned. So no wonder nothing is printed.

% perl -wle 'sub foo { 1 for @_ } print 2 if defined foo(3)'
The loop is run once. So a defined value is returned.

% perl -wle 'sub foo { 1 for @_ } print 2 if foo(3) == 0'
The value is defined but it's numerical value is zeroundef == 0

% perl -wle 'sub foo { 1 for @_ } print 2 if foo(3) eq ""'
and it's an empty stringundef eq ""

I think 1 is different from 43 (or 42 or 2 or whatever other number you can come up besides 0) because it's usually used to represent "true".