my @foo = ('hostname.foo.com 19203949', 'hostname12-2.bar.com 1921202', 'hostname.hi.foo.com 3838313',
    'host12-394.ho.foo.com 31319391');
    
for (@foo){
    my ($sub_dom, $nr) = /^ (.*?) \.foo\.com \s (\d+)/x ;
    print "$sub_dom: $nr\n" if $nr;
}