68 chars, "ab"-compliant too...

sub is_fibo {
  $_=pop;$n=0;{/^|(.)\1*/g?$n^($c=length$&)?0:($n=1-$c+pos,redo):$n>2}
}
[download]

update: 66 chars after deleting an artifact from my previous solution

sub is_fibo {
  $_=pop;$n=1;{/(.)\1*/g?$n^($c=length$&)?0:($n=1-$c+pos,redo):$n>2}
}
[download]

update2: 65 chars, and no more ugly $& usage...

sub is_fibo {
   $_=pop;$n=0;$p=1;{/(.)\1*/g?pos^$n?0:($n+=$p+1,$p=pos,redo):$p>2}
}
[download]

update3: 61 chars, after incorporating ideas from danger's regex and a new method of counting fibonacci's:

sub is_fibo {
  $n=0;pop=~/^((.)(??{"\Q$2"x$n})(?!\2)(?{$n=-++$n+pos})){3,}$/
}
[download]

   MeowChow                                   
               s aamecha.s a..a\u$&owag.print