What does “0 but true” mean in Perl?

$ perl -wle 'print "0 but true" + 3'
3
$ perl -wle 'print "0 but crazy" + 3'
Argument "0 but crazy" isn't numeric in addition (+) at -e line 1.
3

while($c = characters_in_line($file)){
    ...
};

$ strings /usr/lib/perl5/5.10.0/linux/CORE/libperl.so | grep -i true
Perl_sv_true
%-p did not return a true value
0 but true
0 but true

if(x)

for x:        yields:
1             -> true
0             -> false
-1            -> true
"true"        -> true
true          -> true
"false"       -> true
false         -> false # notice the quotes
"0 but true"  ->true   #
int("0 but true") ->false

sub find_x()

if($x = find_x)
{
   print int($x) . "n";
}