I didn't explain myself quite right.

The "/" can be in any part of the string, so a new regexp won't fix it. What I want is to capture four clusters and then, when substituting, replace the "/" inside the clusters.

Right now, I did this:

$do_the_change_mon = sub { 
   $thingy = shift;
   $thingy =~ tr|/|_|;
   return $thingy;
}

$expr = s|
\[([A-Z/]+)\s([A-Z/]+)\s([A-Z/]+)\s([A-Z/]+)\s\]
|&$do_the_change_mon("$1_$2_$3_$4)|xe;
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Thank you all for your kind answers, monks!