...
print "Please enter a number or -1 to end: ";
chomp ( $number = <> );
$smallest = $number; # this is the smallest number we
                     # saw so far
$largest  = $number; # it is also the largest
while ( $number != -1 ) {
    $total   += $number;
    $howmany += 1;
    $smallest = $number if $number < $smallest;
    $largest  = $number if $number > $largest;
    ...
    chomp ( $number = <> );
}
...
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(most programmers would rather do it like Loops said)