Another approach is to ask Perl what it thinks about the code you've written (with further reference to perlop):

>perl -wMstrict -MO=Deparse,-p -le
"my $x = 5;
 $x = $x + ++$x + $x++;
 warn $x;
"
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use strict 'refs';
(my $x = 5);
($x = (($x + (++$x)) + ($x++)));
warn($x);
-e syntax OK

>perl -wMstrict -le
"my $x = 5;
 $x = $x + ++$x + $x++;
 warn $x;
"
18 at -e line 1.
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See O and B::Deparse.