This isn't close to the shortest solution, but I think it's interesting so I'm posting it anyway. :)

sub is_fibo {
my$m=pop;$_=$m=~s/^(.)(?!\1)//for$x,$y;
$m=~s/\G(.)\1*/($z=length$&)==$x+$y||return;$x=$y;$y=$z/ge;
}
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