PassRegex($str =~ /b/i, 'parm2', 'parm3');

I believe that regex is evaluated in list context, not scalar context. In that case, the m operator returns an empty list when there is no match. From perlop:

an empty list is returned upon failure.
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An empty list is a list with no items. So, when there is no match, you actually pass a list of 2 items to your sub, not 3. You could use this to force scalar context:

PassRegex(0+($str =~ /x/i), 'parm2', 'parm3');
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Or, be more explicit with:

PassRegex($str =~ /x/i ? 1 : 0, 'parm2', 'parm3');
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