Add 'use strict' and 'use warnings'. You'll see you're treating 'Users' 'st2641' and 'Desktop' as bareword values, which means Perl doesn't really know, whether they're variables, procedure calls or strings.

I would assume you're aiming to get e.g. "C:\Users\st2641\Desktop\Source_dir". If that's so, you need instead to do:

use strict;
use warnings;

my $volume = "C:";
use File::Spec::Win32;
my $SourceDir      = File::Spec::Win32::catfile( $volume, qw ( Users s
+t2641 Desktop Source_dir  ) ); 

print $SourceDir,"\n";
[download]

Is that more or less what you were after?