$ perl -e 'print 4 + "a"'
4

$ perl -e 'use strict; print 4 + "a"'
4

$ perl -e 'use warnings; print 4 + "a"'
Argument "a" isn't numeric in addition (+) at -e line 1.
4

$ perl -we 'print 4 + "a"'
Argument "a" isn't numeric in addition (+) at -e line 1.
4