This is one of the cases where MO::Deparse can help, by showing how perl interpretes your code:

perl -MO=Deparse -le 'use Time::Piece; $x = localtime; $y = localtime 
+- $x; print $y;'

BEGIN { $/ = "\n"; $\ = "\n"; }
use Time::Piece;
$x = main::localtime();
$y = main::localtime(-$x);
print $y;
-e syntax OK
[download]

localtime - $x is actually interpreted as localtime(-$x)