$ perl -e 'my $m = 20; print $m++ + $m++, "\n"'
41

$ perl -e 'my $m = 20; print $m++ + ++$m, "\n"'
42

$ perl -e 'my $m = 20; print ++$m + $m++, "\n"'
43

$ perl -e 'my $m = 20; print ++$m + ++$m, "\n"'
44