Bottom line - it doesn't! \S matches non-whitespace characters and * means 'grab as many as you can', but if your path or filename contains whitespace characters then things turn a little pear shaped. Consider instead:

use strict;
use warnings;

for my $string (
    '/Unixish path/with/spaces in places/and a file name with spaces',
    'file name only - with spaces',
    '/nice/no/spaces/path/and/filename'
    )
{
    if ($string =~ m!(.*/)?(.*)!) {
        print "Path = '$1', Filename = '$2'\n";
    } elsif ($string =~ /(\S+)/) {
        print "No path, Filename = '$2'\n";
    }
}
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Prints:

Path = '/Unixish path/with/spaces in places/', Filename = 'and a file 
+name with spaces'
Path = '', Filename = 'file name only - with spaces'
Path = '/nice/no/spaces/path/and/', Filename = 'filename'
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In any case you would generally be better to use File::Spec's splitpath.