My forward slash is there. It's just after the filp-flp. No?

No. In your original post, the statement is
    print if (/^\<ReportHost/../^\<\ReportHost>/);
              no forward slash here ^

The  \R escape sequence (matching a generic linebreak) was added with Perl version 5.10. Please see Character Classes and other Special Escapes in perlre. Prior to version 5.10, its use would have earned you an "Unrecognized escape \R passed through..." warning if you had enabled warnings, which you very wisely seem to be doing.

c:\@Work\Perl>perl -wMstrict -le
"print qq{perl version $]};
 ;;
 my $rx = qr{ ^ \< \ReportHost> }xms;
 print $rx;
"
perl version 5.010001
(?msx-i: ^ \< \ReportHost> )


c:\@Work\Perl>perl -wMstrict -le
"print qq{perl version $]};
 ;;
 my $rx = qr{ ^ \< \ReportHost> }xms;
 print $rx;
"
Unrecognized escape \R passed through at -e line 1.
perl version 5.008009
(?msx-i: ^ < ReportHost> )
[download]

Update:

Can you explain or link me to an explaination of the new syntax?

I'm not sure what "new" syntax you're referring to. Documentation for your local installation of Perl should be available to you from the command line via, e.g.,
    perldoc perlre
with the most important regex syntax doc files being perlre, perlretut, and perlrequick. For the  qr// m// s/// tr/// operators, see perlop. (At least I hope you can use perldoc. If not, your Perl installation is seriously b0rken!)

On-line, see The Documentation for all Perl documentation for the most recent and all previous Perl versions.