in reply to sending a request to a web page

Try this. 

my $res = $ua->request(GET
"http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=$BCnumber&db=Nucle
+otide&dopt=xml&txt=on");

[download]

In order to get your variables value substituted into the string, it needs to be interpolated. For this to happen, you need to indicate to Perl that you want it to do ths by using double-quotes rather than single quotes.

Nah! Your thinking of Simon Templar, originally played by Roger Moore and later by Ian Ogilvy

In Section Seekers of Perl Wisdom