Your problem is that that outer map() will return something for each element of @dbs. You probably wanted a grep() there, but even that is problematic.

You should really be thinking "hash" when you want to check if an item is in a set:

sub create_list {
   my @skip_dbs = @_;

   # declare the hash
   my %skippables;

   # populate the hash with the skippables
   $skippables{$_}++ for @skip_dbs;

   my @dbs = qw/fetch forward user smtp/;

   # return a list of each element of @dbs
   # that isn't in the skippables hash
   return grep { ! $skippables{$_} } @dbs;
}

my @list = create_list("user", "smtp");
print "@list";
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