In the example I've shown, the IV slot is set after the first numification.

In fact it doesn't look to me like repeated numification (or magic fetching) was happening.

:~$ perl -MDevel::Peek -E'"42"=~/(\d+)/; say $1; Dump $1; $a=$1+1; Dum
+p $1; say $1; Dump $1' 2>&1 |grep IV
  IV = 0
  IV = 42
  IV = 42
:~$
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Edit

hm ... probably the IV values don't matter if the appropriate I flags aren't set.

~$ perl -MDevel::Peek -E'"42"=~/(\d+)/; say $1; Dump $1; $a=$1+1; Dump
+ $1; say $1; Dump $1' 2>&1 |grep FLAGS
  FLAGS = (GMG,SMG,POK,pPOK)
  FLAGS = (GMG,SMG,IOK,POK,pIOK,pPOK)
  FLAGS = (GMG,SMG,POK,pPOK)
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