Is this you are expecting.
Example-1

my $str="1 2 3 4 590";
my ($a,$b)=split(/ ([^ ]+)$/, $str);
print "a=$a\nb=$b\n";
[download]

Output
a=1 2 3 4
b=590

Example-2

 
my $str="1:2:3:4:590";
my ($a,$b)=split(/:([^:]+)$/, $str);
print "a=$a\nb=$b\n";
[download]

Output
a=1:2:3:4
b=590