I worked on the assumption that your path started with a /. The problem sounded like a reverse regex problem to me so thats how I solved it. The other methods are probably better but what the hey :).

use strict;
use warnings;

my $string = "/base-dir/dir/foo.txt INVALID ORDER-NO 4546090";
my $string2 = "INVALID ORDER-NO 4546090";

print $string,"\n";
$string = join('',reverse(split('', $string)));
$string =~ s/\s*?(\/|-|\w|\.|\d)+$//; 
$string = join('',reverse(split('', $string)));
print $string,"\n\n";

print $string2,"\n";
$string2 = join('',reverse(split('', $string2)));
$string2 =~ s/\s*?\/(-|\w|\.|\d)+$//; 
$string2 = join('',reverse(split('', $string2)));
print $string2;
[download]

This produced:

H:\HTML_T~1\perl>perl regex.pl
/base-dir/dir/foo.txt INVALID ORDER-NO 4546090
INVALID ORDER-NO 4546090

INVALID ORDER-NO 4546090
INVALID ORDER-NO 4546090
[download]

HTH,

Simon