Go step by step. (And, of course, understand each step.)

AFAIU, the original source string is something like '+fo\o*@P@*b\ar+', in which all metacharacters must be metaquoted, but in which there is a '@P@' sub-string that must also be converted into an actual regex pattern. Studying and experimenting with double- and single-quoting rules will be very helpful.

>perl -wMstrict -le
"my $raw = '+fo\o*@P@*b\ar+';
 print qq{'$raw'};
 ;;
 my $mq = quotemeta $raw;
 print qq{'$mq'};
 ;;
 $mq =~ s{ \\ \@ P \\ \@ }'\+([0-9]+)'xmsg;
 print qq{'$mq'};
 ;;
 my $mq_rx = qr{$mq}xms;
 print $mq_rx;
 ;;
 my $s = '123 +fo\o*+987*b\ar+ 456';
 print qq{'$s'};
 $s =~ $mq_rx;
 print qq{'$1'};
"
'+fo\o*@P@*b\ar+'
'\+fo\\o\*\@P\@\*b\\ar\+'
'\+fo\\o\*\+([0-9]+)\*b\\ar\+'
(?msx-i:\+fo\\o\*\+([0-9]+)\*b\\ar\+)
'123 +fo\o*+987*b\ar+ 456'
'987'
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It's also worth remembering that while the  q{} single-quote operator doesn't interpolate, it does do a bit of escaping WRT the \ backslash character, e.g.:

>perl -wMstrict -le
"my $sq = 'start \ \\ \' stop';
 print qq{'$sq'};
"
'start \ \ ' stop'
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