well, glob olny returns the full path if you are doing a search with the full path. if you olny search for *.*, it will show you the current directory. Not useful until you use

chdir("/Library/WebServer/Documents/userpages/$drt/");
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to put you in the right place. Then you can use

glob("*.*");
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to get the files. remember to do this both in the deletion and the selection bit

Alternitivly, get the length of /Library/WebServer/Documents/userpages/$drt/ and in the display section chop it off the front of the string

foreach (@files) {
my $choppedstring=$_;
substr($choppedstring,0,$length_of_dir)="";
print "<br> Delete this File: $choppedstring <INPUT TYPE=\"checkbox\" 
+NAME=\"files\" VALUE=\"$_\">\n";
}
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Usernames aren't a great idea to pass around (if people don't know what a vaild user name is, then thats another step they need to take in breaking your security), although I'm not sure what another solution is.
as for the regex's, I'm not sure. They've never been a strong point for me.