Technically, it's possible to tell the difference 

>perl -MDevel::Peek; -e"Dump(0x007102)"
SV = IV(0x18265fc) at 0x2252f8
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,IOK,READONLY,pIOK)
  IV = 28930

>perl -MDevel::Peek; -e"Dump(qq{\x30\x05\x15})"
SV = PV(0x226680) at 0x225300
  REFCNT = 1
  FLAGS = (PADBUSY,PADTMP,POK,READONLY,pPOK)  <-- Not IOK/UOK/NOK
  PV = 0x182f6a4 "0\5\25"\0
  CUR = 3
  LEN = 4

[download]

But checking for that would be extremely fragile. I strongly recommend against that approach. Instead, use some kind of meta data (a different command line switch, for example) to specify the format, then canonize it for internal use. 

# Inputs
my $input = "\x30\x05\x15";
my $packed = 1;

# Canonize input
if ($packed) {
   $input = unpack('N', substr(("\x00"x4).$input, -4));
}

# Do some work
#...

# Output results
printf("Results for 0x%X are %s\n", $input, "...");

[download]

Update: Added second snippet.

In reply to Re^3: Differences in behavior in pack by ikegami
in thread Differences in behavior in pack by sxmwb

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