After a good nights sleep I looked more thoroughly using Devel::Peek as you pointed me to.
And now I think we're closing in.

In the test script:

Dump($v2); ## prints
SV = PV(0x176ec38) at 0x1765274
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x17601c0 "3.25"\0
  CUR = 4
  LEN = 8

my $v3 = 2.5 * $v2;
Dump($v3); ## prints
SV = NV(0x1761830) at 0x177ee88
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,pNOK)
  NV = 8.125
[download]

BUT with the original program

Dump($v2); ## prints
SV = PV(0x1b69008) at 0x1b87164
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK)
  PV = 0x1b6ab58 "3.25"\0
  CUR = 4
  LEN = 5

my $v3 = 2.5 * $v2;
Dump($v3);  ## prints
SV = NV(0x178ecbc) at 0x1b87218
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,NOK,pNOK)
  NV = 7,5
[download]

From this I gather that the problem has something to do with the decimal delimiter, maybe the locale or somesuch has been changed under my feet. The difference between the original script and the test is that the original uses DBI

I will now upgrade my Activestate 623 distribution to 631 to see if this resolves the problem

More ideas ?

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It's unfair to be an expert.