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?

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

If you look again at the original post, you’ll see that the forward slash which is there is functioning as a delimiter for the regex. But the forward shash to be matched is missing. That’s why I recommended using different delimiters for the regex: if the delimiter is something other than a slash, you don’t need to backslash the slash inside the regex. Too many back- and forward-slashes produce what is called “leaning toothpick syndrome.”

On the syntax for delimiters, see, e.g., perlretut#Simple-word-matching and perlop#Quote-and-Quote-like-Operators. Note that if you use any delimiter other than a forward slash, you need to prefix the regex with an m: /abc/ becomes m!abc! or m[abc], etc.

Hope that helps,