As was stated above, you're having problems with operator precedence. Consider the following two one-liners:

perl -le '$a = undef or 1; print $a'             
perl -le '$a = undef || 1; print $a'
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You may have to change the single-quotes to double-quotes to get the above examples to run from the command line, depending on what platform you're on

As perlop tells us, 'or' has lower precedence than '=', while '||' has higher precedence than '='. So the first one-liner is equivalent to:

perl -le '($a = undef) or "foo"; print $a'
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whereas the second one is equivalent to:

perl -le '$a = (undef or "foo"); print $a'
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