HI,
I figured out the way to make the regular changes to the URLs, I am giving a set of URLs which are actually in an array. I guess i can use  foreach $url (@urls) {} to get each URL. But i need to save the output in a file, such that each download is appended to the same file, so at the end of the run, i have a single file, which I can manipulate! Is is possible.
I have gone through the POD doccumentation for the LWP::UserAgent and LWP::Simple, but not being a trained programmer, I really couldnt make a lot of sense out of it. Below is the set of URLs that i generated

http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_021973.16&from=1
+428112&to=1428312&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_006713.13&from=1
+7592746&to=17592946&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_033899.6&from=30
+911489&to=30911689&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_008183.17&from=5
+688508&to=5688708&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_008413.16&from=1
+2431428&to=12431628&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_010194.16&from=4
+0748608&to=40748808&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_011512.9&from=24
+538334&to=24538534&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_008583.16&from=2
+0870594&to=20870794&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_007933.13&from=4
+3842762&to=43842962&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_004836.15&from=1
+0944910&to=10945110&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_007592.13&from=5
+267636&to=5267836&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_026446.12&from=1
+605346&to=1605546&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_028413.7&from=42
+4829&to=425029&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_011109.15&from=4
+075427&to=4075627&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_005058.14&from=1
+6657433&to=16657633&txt=on&view=gb
http://www.ncbi.nlm.nih.gov/entrez/viewer.fcgi?val=NT_033899.6&from=39
+71767&to=3971967&txt=on&view=gb
[download]

Thanks in advance for your help
Sameet