The  m// operator returns false (the empty string), not undef, on match failure when evaluated in scalar context:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $str = 'abc';
 PassRegex(scalar($str =~ /x/i), 'parm2', 'parm3');
 ;;
 use Data::Dumper;
 ;;
 sub PassRegex {
   print Dumper(\@_);
   print qq{BoolExpr = '$_[0]'  parm2 = '$_[1]'  parm3 = '$_[2]'};
   return;
   }
"
$VAR1 = [
          '',
          'parm2',
          'parm3'
        ];

BoolExpr = ''  parm2 = 'parm2'  parm3 = 'parm3'
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Please see Regexp Quote-Like Operators.