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 sub _is_text {
     my ($content_type) = @_;
-    return $content_type =~ /(x(?:ht)?ml|text|json|javascript)/;
+    return $content_type =~ /(\bx(?:ht)?ml\b|text|json|javascript)/;
 }

my $buffer;
open my $fh, '>', \$buffer;
...
    #content_type => 'application/vnd.ms-excel',
    filename     => 'xxx.xls',
);