$ perl -le '$x="12345678901234567890"; print $x; ++$x; print $x'
12345678901234567890
12345678901234567891

$ perl -e '$x="12345678901234567890";printf "%20.0f\n",$x; ++$x;printf "%20.0f\n",$x'
12345678901234567168
12345678901234567168

# or, just as bad:
$ perl -le '$x="12345678901234567890"; printf "%20.0f\n",$x; ++$x; print $x'
12345678901234567168
1.23456789012346e+19