To return no value from a map, you're probably going to need to explictly return a null list, ():

my @map = 
  map { $_->{'username'} eq $cgi->param('username') ? $_ : () } @{%{$x
+ml}->{'user'}};
[download]

However this is better written with grep:

my @grep = grep { $_->{'username'} eq $cgi->param('username') } @{%{$x
+ml}->{'user'}};
[download]