B::Deparse (Tip #6 from the Basic debugging checklist) shows that <> in your 1st example is indeed a glob, but <> in the 2nd example is not a glob:

1st:

use File::Glob ();
use warnings;
@FILES = glob('//server/path/WantedFiles*.*');
foreach $file (@FILES) {
    ();
}
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2nd:

$WantedFiles = '//server/path/WantedFiles*.*';
@FILES = <$WantedFiles>;
foreach $file (@FILES) {
    ();
}
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I/O Operators