I think CGI implements this behavior so that param() can support both scalar and list context. In list context it returns all the values for a parameter and in scalar context it just returns the first value.

If it returned undef for unknown parameters, then this wouldn't work:

my @vals = $q->param('...');
print Dumper(\@vals); # would print [ undef ] not [ ]
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That is, @vals would not be the empty list but a list containing undef.

A work-around is to use scalar:

my $var = { first  => scalar $q->param('first'),
            second => scalar $q->param('second'),
          };
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