I was just browsing the perldocs when I came across this in an example ( http://perldoc.perl.org/perlre.html#Regular-Expressions - see Capture Groups example )
"aa" =~ /${a}/; # True
"aa" =~ /${b}/; # True
"aa0" =~ /${a}0/; # False!
"aa0" =~ /${b}0/; # True
"aa\x08" =~ /${a}0/; # True!
"aa\x08" =~ /${b}0/; # False
I couldn't find any documentation on what that syntax means.
So what does the regex /${a}/ mean in this context?
$ with brackets avoid the ambiguity of variable names. Such that:
$foo = 'house';
'housecat' =~ /$foo/; # matches
'cathouse' =~ /cat$foo/; # matches
'housecat' =~ /${foo}cat/; # matches
Also in the link that you have given, there is a definition for $a and $b, but you have forgotten to copy here.