print "$def\n" if ! $val or $val =~ /^0|1$/;
Note that !$val already covers the case where $val is 0.
Also /^0|1$/ is not what you might think it is. The OR | in regexes has a looser precedence than concatenation, which means the regex is the same as ^0 OR 1$, so it also matches strings like 0foo or bar1.
The correct way to write it is /^(0|1)$/, or if you don't want to create a capture, /^(?:0|1)$/.
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