+class don't group or capture. Be aware that, unless the pattern is
+evaluated in single-quotish context, variable interpolation will take
+place before the bracketed class is parsed:
+
+ $, = "\t| ";
+ $a =~ m'[$,]'; # single-quotish: matches '$' or ','
+ $a =~ q{[$,]}' # same
+ $a =~ m/[$,]/; # double-quotish: matches "\t", "|", or " "