use strict;
use warnings;

my $line;
while (<DATA>) {
     chomp;
     if (/\[(server\d+)\]/) {
     	print $line, "\n" if defined $line;
     	$line = $1 . ": ";
     } else {
	     $line .= $_;
	     $line .= ',';
     }
}
print $line, "\n";

__DATA__
[server1]
/tmp/location1/file.log
/tmp/location2/file.log
[server2]
/usr/loc1/file.log
/usr/loc2/file.log
[server3]
/citrix/dir3/file.log