Whitout the parens you map is *greedy* and thus eats argument 4 ie the digit 1. There is a good chance you have no cgi param '1' so this becomes undef.

use CGI;
my $q = CGI->new( { one => 1, two => 2, three => 3 } );
test(   map{$q->param($_)||"${_}_IS_NULL"}qw(one two three)  , 4 );
test( ( map{$q->param($_)||"${_}_IS_NULL"}qw(one two three) ), 4 );

sub test { $"=', ';print "Args( @_ )\n" }

__DATA__
Args( 1, 2, 3, 4_IS_NULL )
Args( 1, 2, 3, 4 )
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Your obvious expectation is that the qw( ) perfoms a useful binding function to the map when in fact you are getting a flat list: func( map{$q->param($_)} 1, 2, 3, 4 )