This check has always been highly fragile (sorry for the Windows command line quoting):

perl -Mutf8 -e "$x = qq(42); if( 0+$x ) {}; if ($x ^ $x) {print qq(str
+ing\n);}"
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That oneliner outputs nothing thus considers $x not a string even if $x was never "modified". A simple check of a variable can change what "value type" your test considers that variable to contain.