Maybe it's a CPU cache thing. I'm just guessing because I expect the code path you're benchmarking to be identical.
I would guess it is because both the number AND the string representation are copied: 

 perl -MDevel::Peek -e'$x=42; "$x"; $y = $x; Dump $y'
SV = PVIV(0x814e9a8) at 0x814d518
  REFCNT = 1
  FLAGS = (IOK,POK,pIOK,pPOK)
  IV = 42
  PV = 0x8168e70 "42"\0
  CUR = 2
  LEN = 4

[download]
In summary - the slowdown effect should only be noticable when you're copying the variables: as you implied, using arithmatic on numbers that are also stringified should not slow down (much).

update: Ieronim, the issue is that when you stringify a number, perl stores both representations in the variable.

When you copy it, perl doesn't know whether you're going to use the copies as string or numbers, so it copies the whole variable (since that should be faster than copying only one representation and converting it later).

"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re^2: Don't treat your numbers as strings, or Interpolation is worse than you might think by Joost
in thread Don't treat your numbers as strings, or Interpolation is worse than you might think by Ieronim

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