Basically, any string can be used as a regex. Vis.

print 'Matched!' if 'fred' =~ 'fred'
Matched!
[download]

Or perhaps more bizarrly,

print 'Matched!' if '    ' =~ pack 'V', 538976288
Matched!
[download]

Whether an arbitrary string will survive compilation and do anything useful, or what it will do if it does is a harder question to answer.

Question: Would a legless duck be a free floating spirit? :)