tilde:~ [13:53:30]$ perl -we '$a=$a+1'
Use of uninitialized value in addition (+) at -e line 1.
tilde:~ [13:53:50]$ perl -we '$a+=1'
Name "main::a" used only once: possible typo at -e line 1.
tilde:~ [13:53:58]$ perl -we '$a++'
Name "main::a" used only once: possible typo at -e line 1.