"Mac OS" ne "OS X". Mac OS has ":" as path delimiter, OS X is derived from Unix and thus has a "/" as path delimiter.

File::Basename can change its idea of the operating system, so you can see the problem on every operating system:

use File::Basename;

my $url = "http://google.com/folder/1234.html";

                               print "Native: ",basename($url,'.html')
+,"\n";
fileparse_set_fstype('Unix');  print "Unix: ",  basename($url,'.html')
+,"\n";
fileparse_set_fstype('MacOS'); print "MacOS: ", basename($url,'.html')
+,"\n";
[download]

Output on Windows:

Native: 1234
Unix: 1234
MacOS: //google.com/folder/1234

Abusing that feature to force Unix semantics doesn't really help:

use File::Basename;

my $url = "http://google.com/folder/1234.html?session=4247110815";

                               print "Native: ",basename($url,'.html')
+,"\n";
fileparse_set_fstype('Unix');  print "Unix: ",  basename($url,'.html')
+,"\n";
fileparse_set_fstype('MacOS'); print "MacOS: ", basename($url,'.html')
+,"\n";
[download]

Output:

Native: 1234.html?session=4247110815
Unix: 1234.html?session=4247110815
MacOS: //google.com/folder/1234.html?session=4247110815

Alexander