I'm too lazy to work out a complete solution but a clean approach would be to deparse the regex-compilation:

> perl -e 'use re 'debug';/(?i-xsm:(?<mon>\w+)\s+(?<day>\d+)\s+)/' 
Compiling REx "(?i-xsm:(?<mon>\w+)\s+(?<day>\d+)\s+)"
Final program:
   1: OPEN1 'mon' (3)
   3:   PLUS (5)
   4:     ALNUM (0)
   5: CLOSE1 'mon' (7)
   7: PLUS (9)
   8:   SPACE (0)
   9: OPEN2 'day' (11)
  11:   PLUS (13)
  12:     DIGIT (0)
  13: CLOSE2 'day' (15)
  15: PLUS (17)
  16:   SPACE (0)
  17: END (0)
stclass ALNUM plus minlen 4 
Freeing REx: "(?i-xsm:(?<mon>\w+)\s+(?<day>\d+)\s+)"
[download]

Now fetching all /OPEN\d+ '(\w+)'/-opcodes shouldn't be too difficult.

See perldoc re for more options.

UPDATE: shrank example.