seems to me there's a limitation in the current lexer approach which doesn't allow to tokenise interpolated variables within strings which is a common idiom in scripting languages like perl, php or bash. so these variables cannot be rendered with another visual style. this is not a bizarre request. there are several editors capable of highlighting interpolated vars, like vim, kate, nedit.
It works for PHP but not Perl or Shell scripts (from my testing). I guess probably file a feature request[1] with the Scintilla project and it will trickle into Geany when Scintilla's lexers have this feature.
[1] https://sourceforge.net/tracker/?group_id=2439&atid=352439