looks like a college assn
but who cares

while(print "in> " and chomp($a = <STDIN>) and $a >= 0){
    $c++;
    $s += $a;
    $m = $a if !$m;
    $M = $a if $M < $a;
    $m = $a if $m > $a;
}

print "max: $M\navg: ".$s/$c."\nmin: $m\n";
[download]

here is another one
a bit uglier and shorter

push @r, $a and $s += $a while print "in> " and chomp($a = <STDIN>) an
+d $a >= 0;
@r = sort {$a cmp $b} @r;
print "max: $r[$r-1]\navg: ".$s / @r." \nmin: $r[0]\n";
[download]