If you have Perl version 5.10+ (update: and I see now that you use the  /r modifier on the  s/// in one of your OPed code examples, so you must have at least 5.14), the Extended Patterns  (?(condition)yes-pattern) construct where condition is  (?{ CODE }) may be of interest as it doesn't have so much scary "experimental feature" stink on it. 

c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 for my $s (qw(aaxyxyxy aaaxyxyxy aaaaxyxyxy)) {
   my $match = $s =~ m{
     \b (a+) ((?: xy)+) \b
     (?(?{ 2 * length($1) != length($2) }) (*FAIL))
     }xms;
   print $match ? '   ' : 'NO ', qq{match '$s'};
   }
"
NO match 'aaxyxyxy'
   match 'aaaxyxyxy'
NO match 'aaaaxyxyxy'

[download]
To a very limited degree, you can get away from absolute-capture numbering with tricks like this: 
c:\@Work\Perl\monks>perl -wMstrict -le
"use 5.010;
 ;;
 for my $s (qw(aaxyxyxy aaaxyxyxy aaaaxyxyxy)) {
   my $match = $s =~ m{
     \b (a+) (?{ length $^N }) ((?: xy)+) \b
     (?(?{ 2 * $^R != length $^N }) (*FAIL))
     }xms;
   print $match ? '   ' : 'NO ', qq{match '$s'};
   }
"
NO match 'aaxyxyxy'
   match 'aaaxyxyxy'
NO match 'aaaaxyxyxy'

[download]
And then there are named captures. See also Special Backtracking Control Verbs for (*FAIL).

Give a man a fish:  <%-{-{-{-<

In reply to Re: interpolating code in regex brace quantifier by AnomalousMonk
in thread interpolating code in regex brace quantifier by grobber

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