What's the difference between $heap->{127.2}{127.1.0.2} and $heap->{'127.2'}{'127.1.0.2'} ?

sunshine_august has asked for the wisdom of the Perl Monks concerning the following question:

Hi, all monks:

I encounted a strange problem with hash in Perl, I have write a tiny script to demostrate this problem:

#!/usr/bin/perl
 use strict;
 use warnings;

 my $heap = {};

 $heap->{'172.16'} = 1;
 print "\$heap->{'172.16'}: $heap->{'172.16'}\n";
 print "\$heap->{172.16}: $heap->{172.16}\n";

 $heap->{'127.2'}{'127.1.0.2'} = 2;
 print "\$heap->{'127.2'}{'127.1.0.2'}: $heap->{'127.2'}{'127.1.0.2'}\
+n";
 print "\$heap->{127.2}{127.1.0.2}: $heap->{127.2}{127.1.0.2}\n";
[download]

and here is the executed result:

$heap->{'172.16'}: 1
$heap->{172.16}: 1
$heap->{'127.2'}{'127.1.0.2'}: 2
Use of uninitialized value in concatenation (.) or string at /home/lar
+ry/project/sysmonitor/test/testHash2.pl line 28.
$heap->{127.2}{127.1.0.2}:
[download]

What surprise me is that $heap->{'127.2'}{'127.1.0.2'} is not the same as $heap->{127.2}{127.1.0.2}.

Can anyone give some tips?

Comment on What's the difference between $heap->{127.2}{127.1.0.2} and $heap->{'127.2'}{'127.1.0.2'} ? Select or Download Code

Replies are listed 'Best First'.
Re: What's the difference between $heap->{127.2}{127.1.0.2} and $heap->{'127.2'}{'127.1.0.2'} ? by tilly (Archbishop) on Dec 22, 2008 at 08:57 UTC
127.1.0.2 is being interpreted as a version string. Search for "Version Strings" in perldata for details.	[reply]
Re^2: What's the difference between $heap->{127.2}{127.1.0.2} and $heap->{'127.2'}{'127.1.0.2'} ? by AnomalousMonk (Archbishop) on Dec 22, 2008 at 16:44 UTC
Most particularly, see the first paragraph in the sub-section on 'Version Strings' in the section Scalar value constructors of perldata: Note: Version Strings (v-strings) have been deprecated. They will be removed in some future release after Perl 5.8.1. The marginal benefits of v-strings were greatly outweighed by the potential for Surprise and Confusion.	[reply]
Re^3: What's the difference between $heap->{127.2}{127.1.0.2} and $heap->{'127.2'}{'127.1.0.2'} ? by Anomynous Monk (Scribe) on Dec 23, 2008 at 02:28 UTC
And pre-version-strings it still would have not been the same as '127.1.0.2', since it would have been interpreted as (127.1) . (0.2), giving a key of '127.10.2'.	[reply]
Re: What's the difference between $heap->{127.2}{127.1.0.2} and $heap->{'127.2'}{'127.1.0.2'} ? by Bloodnok (Vicar) on Dec 22, 2008 at 13:26 UTC
Further to tillys observation & recommendation, the general case is that if, when explicitly used, the name of the key contains a character in `\W`, then the key name requires enclosing in quotes in order that perl doesn't mis-interpret the actual meaning of the key name ... as tilly observes in this case. A user level that continues to overstate my experience :-))	[reply] [d/l]
Re: What's the difference between $heap->{127.2}{127.1.0.2} and $heap->{'127.2'}{'127.1.0.2'} ? by Anonymous Monk on Dec 22, 2008 at 08:41 UTC
Start your program with `perl -MO=Deparse,-p foo.pl` [download] and compare the output to foo.pl	[reply] [d/l]
Re^2: What's the difference between $heap->{127.2}{127.1.0.2} and $heap->{'127.2'}{'127.1.0.2'} ? by sunshine_august (Scribe) on Dec 22, 2008 at 09:21 UTC
the output like this: `use warnings; use strict 'refs'; (my $heap = {}); ($$heap{'172.16'} = 1); print("\$heap->{'172.16'}: $$heap{'172.16'}\n"); print("\$heap->{172.16}: $$heap{172.16}\n"); ($$heap{'127.2'}{'127.1.0.2'} = 2); print("\$heap->{'127.2'}{'127.1.0.2'}: $$heap{'127.2'}{'127.1.0.2'}\n" +); print("\$heap->{127.2}{127.1.0.2}: $$heap{127.2}{127.1.0.2}\n");` [download] and have the same executed result: `$heap->{'172.16'}: 1 $heap->{172.16}: 1 $heap->{'127.2'}{'127.1.0.2'}: 2 Use of uninitialized value in concatenation (.) or string at testHash3 +.pl line 25. $heap->{127.2}{127.1.0.2}:` [download]	[reply] [d/l] [select]