Sadly this will fail for any movement of greater than 9 points. This problem is insoluble without a definition of what constitutes a point. Consider the upper number pair and some possible variations that could exist with trailing zero truncation:

1.5553 - 1.5552 # 1 point
1.5553 - 1.55   # 53 points (your sub will fail with this)
1.56   - 1.55   # 1 point? 100 points? (insoluble from data alone)
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You either have to decide what is a point, or guess and accept that there will be errors:

sub points {
    my ($was,$is,$exp) = @_;

    unless ($exp) {
        $was =~ m/\.(\d+)$/;
        my $x = $1 ? length($1) : 0;
        $is =~ m/\.(\d+)$/;
        my $y = $1 ? length($1) : 0;
        $exp = $x > $y ? $x : $y;
    }
    print "($exp) $was => $is\t";
    $exp = 10**$exp;
    my $dif = $is*$exp - $was*$exp;
    # add correction factor to allow int to round correctly
    # also ensures that FP "error" such as 4.999999 ends up as +5
    $dif += $dif < 0 ? -0.5 : +0.5;
    return sprintf "%+d", $dif;
}
print points(1.5553,1.5552), $/;
print points(1.55,1.5553 ),  $/; 
print points(1.55,1.56),     $/;
print points(1.55,1.56,4),   $/;  
print points(0.9984,0.998),  $/;

__DATA__
(4) 1.5553 => 1.5552    -1
(4) 1.55 => 1.5553        +53
(2) 1.55 => 1.56        +1
(4) 1.55 => 1.56        +100
(4) 0.9984 => 0.998        -4
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