my $str = 'xyzxyzxyz - Prod Cache Broker - 16100';
print "$1\n" if ($str =~ m/^(\S+)\s+?.+$/);
[download]

The problem you were having is that (.+) is greedy, so it is consuming "xyzxyzxyz - Prod Cache Broker - 16100", and then the \s+? was forcing it to give up the space immediately preceding "16100", and finally the .+ was grabbing "16100".

If you wanted the parens to capture only up to (but not including) the first space, just tell it to match non-space characters.