Is there a way to have map return no value (compared with null value) into the output array if a given condition on the input scalar is not met?

Well, yes. I think $foo eq $bar ? $_ : () would do it (among other possibilities). But why not just use grep instead?

my @grepped = grep {$_->{'username'} eq $cgi->param('username')} 
                @{ %{$xml}->{'user'} };
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If God had meant us to fly, he would *never* have given us the railroads.
    --Michael Flanders