The point is that in:
print "P1 has path\n" if ($P1 =~ /$RE{delimited}{ -delim => '\\\/' }/ );
the quoted string becomes
-delim => '\/' ;
In:
print "P1 has path\n" if ($P1 =~ /$RE{delimited}{ -delim => '\/' }/ );
The quoted string becomes:
-delim => '/' ;
This is because the \ is treated as an escape character beteween //.

I disagree. In the index expression of an array (positional or associative), the expression is evaluated in scalar context and not in the double-quotish context of a regex into which the array element may happen to be interpolated. So  '\\\/' and  '\/' are evaluated in single-quotish context and become the character sequences  \\/ and  \/ respectively. And because of the way backslashes are interpreted in single-quote context,  '\\\\/' and  '\\\/' are equivalent, and  '\\/' and  '\/' likewise. E.g.:

c:\@Work\Perl\monks\Veltro>perl -wMstrict -MData::Dump -le
"my %RE = (
   '\\\\/' => 'BackBackFwd1',
   '\\\/'  => 'BackBackFwd2',
   '\\/'   => 'BackFwd1',
   '\/'    => 'BackFwd2',
   '/'     => 'Fwd',
   );
 dd \%RE;
 ;;
 my $rx = qr{ $RE{'\\\\/'} $RE{'\\\/'} $RE{'\\/'} $RE{'\/'} $RE{'/'} }
+;
 print $rx;
"
{ "/" => "Fwd", "\\/" => "BackFwd2", "\\\\/" => "BackBackFwd2" }

(?^: BackBackFwd2 BackBackFwd2 BackFwd2 BackFwd2 Fwd )
[download]

There are a couple of Data::Dump::dd() and hash peculiarities:

• Why is the key of the value  "BackBackFwd2" in the dd dump represented as  "\\\\/" when it's given as  '\\\/' in the hash definition? This is an artifact of the way dd represents strings only as double-quoted strings, so a single backslash can only be literally defined as the  "\\" escape sequence.
• Why is there no  'BackBackFwd1' value in the hash? Because the  '\\\\/' and  '\\\/' string literals compile identical character sequences (update: i.e., identical keys), and the second key (with the value 'BackBackFwd2'.) supersedes the first. (And likewise with 'BackFwd1')

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