style-guide: add guidelines for standard streams; fix remaining pages (#11196)

* readonly, {un,}compress, zbarcam: use proper syntax for IO streams

* style-guide: add guidelines for standard streams

* duckdb, perl: add backticks around stdin and stdout

pages/common/perl.md

@@ -3,7 +3,7 @@
 > The Perl 5 language interpreter.
 > More information: <https://www.perl.org>.
 
-- Print lines from stdin [m/] matching regex1 and case insensitive [/i] regex2:
+- Print lines from `stdin` [m/] matching regex1 and case insensitive [/i] regex2:
 
 `perl -n -e 'print if m/{{regex1}}/ and m/{{regex2}}/i'`