use strict;
use warnings;
use feature qw/ :5.10 /;

my @str = ("j    k l         foobar", "foobar", "jkl foobar", "1 2 3", "jk l foobar", "foobar j k l", "foobar j kl", "           ", " ", "j jk foobar", "j k jk foobar", "j k l");

my $rx = qr/^((?:\w(?:\s+|\z))*)(.*)/;

for my $str (@str){
  my ($i,$s) = $str =~ /$rx/;
  $i =~ s/\s+//g;
  say $i ? join " ",$i,$s : $s;
}