use re 'debug'
Causes Perl to produce debugging messages when compiling and at run time when using regular expressions. See the re manpage and the section on "Debugging regular expressions" in the perldebug manpage for details.
use re 'eval'
Permits a regular expression to contain
assertion even if the regular expression contains variable interpolation,
which normally isn't permitted for security reasons. The pragma is ignored
if the regular expression comes from tainted data.
use re 'taint'
When a tainted string is the target of a regular expression, causes
the regular expression memories (or
the values returned by the
m// operator in list context) to be