http://www.ncbi.nlm.nih.gov/mapview/seq_reg.cgi?org=human&chr=7&from=1&to=158545518&cntg=cntg

Okay...that's a start. First, let's cover the "random sequence" part. I'll make some assumptions here, as I don't really have a lot to go on. I'll assume that you have your data in a single string (I'll call it $super). So, in order to get a "random sequence", there are two elements of randomness that we have the ability to introduce: length and starting position. I'll try to incorporate both.

my $start = int(rand(length($super)));
my $length = int(rand(length($super) - $start)) + 1;
my $substring = substr($super, $start, $length);
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Of course, you could make either $start or $length static by just assigning a value to them.

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Now, on the the fetching part. Once you have all of the appropriate values for the variable in your URL, fetching it is not so bad:

use LWP::Simple;

getstore($url, $localfile);
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If I can elaborate any further, let me know how and I'll do my best.

thor