Allthough, this is repetitive, using one-liners like:

perl -e "use re 'debug'; print 'this is a demo' =~ /\w+/;"
[download]

helped me more than once (it's even more useful with complex regexes :-). It'a a shame that you have to dig into perldebug to find it. I think it should be mentioned in perlre also.