To convert the first string, (?smi)^STOR\s[^\n]{100}, into a compiled regular expression for P::RG::R, just use qr/$string/, assuming $string contained the string as read from the external source (don't forget to chomp it if that's the case!).
To convert the second string, /^STOR\s[^\n]{100}/smi, again, assuming $string is read from an external source (not perl code - perl DATA is fine), just use eval "qr$string".
No matter what, if the string is starting out inside perl code, you have to escape the \'s that have special meaning to the regular expression parser but not to perl, such as \s, \w, \S, \W, \d, \D, ... so that they survive to the parser. This is not needed if the string is stored outside of code because then the perl compiler won't see the \'s.