That use of $; is very clever, as it parses correctly and allows you to omit the brackets for the for. Combining this with other modifications yields 122 chars of obfuscated goodness:

print map{$i=$_/20;$/,map{$r=$;=$_/20;$z=$i;$n=$z*$z,$z=2*$r*$z+$i,$r=($m=$r*$r)-$n+$;for a..z;$m<4?$":"+"}-46..13}-20..19

Further savings if you choose 'X' instead of '+':

print map{$i=$_/20;$/,map{$r=$;=$_/20;$z=$i;$n=$z*$z,$z=2*$r*$z+$i,$r=($m=$r*$r)-$n+$;for a..z;$m<4?$":X}-46..13}-20..19

Here is the original C code, just for comparison, at 182 characters:

void main(){double O,x,o,I,l,i;char _;for(l=-1;l<1;l+=.05,putc
;i<.7;i+=.05,putchar(_?'?':':'),O=i,x=l,o=I=(_=1)--)for(;++_&&
);x=2*O*x+l,O=o-I+i);}
[download]

C allows for shorter variable names, and Perl still comes out ahead. Crazy.