What is the tr/!-~/P-~!-O/ doing?

tr is perl's transliteration function.

It translates the characters in the string it is applied to, according to the two tables of characters given as its arguments.

For $string =~ tr/123/ABC/, it would change any and all occurrences of the characters '1','2', or '3', for the characters 'A', 'B', and 'C' respectively. Ie. If $string contained "a1b2c3c3b2a1", after the above code it would then contain "aAbBcCcCbBaA".

In your example, the first list is specified as: !-~ which is a short-hand notation meaning all the characters between '!' and '~', That is all the visible characters in the (7-bit) ASCII character set. Eg:

!"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdef
+ghijklmnopqrstuvwxyz{|}~
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The second list, P-~!-O specifies the same characters, but in a different order, as two ranges. 'P' through '~' and '!' through 'O'. Effectively,

PQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~!"#$%&'()*+,-./01234567
+89:;<=>?@ABCDEFGHIJKLMNO
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The effect of these tables is that '!' will be translated to 'P' (and 'P' will become '!'); '"' will become 'Q' (and 'Q' becomes '"') and so on.

This is a reversible (obfuscation) process similar to ROT13. Effectively "ROT47".

However, why this is being applied to the input data, your guess is as good as mine. Probably better :)

This was so strange I just had to know. It turns out that ROT47 is how OpenSolaris hides the hostid string from... well hardly anyone who wanted to know really. The second link below is a rather frank discussion of the security through obscurity approach of maintaining a fundamentally broken licence control scheme.
See http://hub.opensolaris.org/bin/view/Community+Group+on/2008092701
and http://arc.opensolaris.org/caselog/PSARC/2007/078/hostid.opensolaris.txt

--
I'd like to be able to assign to an luser

Awesome! Thanks for your reply!

#!/usr/bin/perl

use strict;
use warnings;
use Data::Dumper::Concise;

my $string = '0x0ddb00b5';
my $rot = rot47($string);
print Dumper($rot);

sub rot47 {
  my $str = shift;
  $str =~ tr/!-~/P-~!-O/;
  return $str;
}
exit 0;
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"_I_553__3d"
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I translated the regexp into simple perl

What regexp?    There is no regular expression in either the question or your reply.

Thanks for the effort! I didn't know rot47 is a encryption method of ASCII text file.

I didn't know rot47 is a encryption method of ASCII text file.

s/encryption/obsfucation/. This is a very weak method that is not secure from anyone but the most honest of users.

--MidLifeXis

echo "0x0ddb00b5" | /usr/bin/perl -e 'print("\""); while(<STDIN>){chop
+;tr/!-~/P-~!-O/;print $_;} print("\"\n"); exit 0;'
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That could be written more simply as:

echo "0x0ddb00b5" | /usr/bin/perl -lne'tr/!-~/P-~!-O/; print qq/"$_"/'
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Thanks:-)