When I play the role of a human explaining a number, I tend to round up the absolute value at some fuzzy index of that number. After the round-up, anything that follows that index is zero-ed out.
sub fuzz {
    my $n = shift();
    return unless defined($n);

    my $neg = ($n < 0);
    ($n) = ($n =~ /(\d+)/);
    $n += 0;
    my ($c1, $c2) = ($n =~ /^(\d{0,2}\d{3})((?:\d{3})*)$/);

    return $neg ? -$n : $n unless defined($c1);

    my $c3 = substr($c1, -3, 2, "");
    my $c4 = substr($c1, -1, 1, "");
    my $n2 = $c1 . $c3;

    my $x1 = 100 - $c3;
    my $x2 = (10 - $c3 % 10) % 10;
    $n2 += $c1
      ? (
          ($x1 <= 30 && $c1 >= 10) ? $x1
          : $x2 ? $x2
          : $c4 ? 1 : 0
        )
      : ($c4 ? 1 : 0);
    $n2 .= 0 x (length($c2) + 1);

    return $neg ? -$n2 : $n2;
}

[download]

The tests explain better than words:

use Test::More;

# same behavior for -ve numbers as +ve ones
is( fuzz(-721), -730 );
is( fuzz(-98), -98 );
is( fuzz(-2), -2 );

# fractions ignored
is( fuzz(-0.5), 0 );
is( fuzz(0.005), 0 );
is( fuzz(0.9), 0 );

# up to 2 digits returned as-is
is( fuzz(0), 0 );
is( fuzz(3), 3 );
is( fuzz(9), 9 );
is( fuzz(96), 96 );

# 3 digits
is( fuzz(120), 120 );
is( fuzz(123), 130 );
is( fuzz(721), 730 );
is( fuzz(809), 810 );
is( fuzz(980), 980 );
is( fuzz(993), 1000 );

# 4 digits
is( fuzz(1234), 1300 );
is( fuzz(1720), 1800 );
is( fuzz(3990), 4000 );
is( fuzz(9910), 10000 );

# 5 digits
is( fuzz(18433), 18500 );
is( fuzz(21250), 21300 );
is( fuzz(21750), 22000 );
is( fuzz(99800), 100000 );

# 6 digits follow 3-digit behavior
# 7 digits follow 4-digit behavior and so on
is( fuzz(123_555), 130_000 );
is( fuzz(219_900), 220_000 );
is( fuzz(633_999), 640_000 );
is( fuzz(1_234_567), 1_300_000 );
is( fuzz(99_402_888), 99_410_000 );
is( fuzz(99_611_222), 99_700_000 );
is( fuzz(99_704_567), 100_000_000 );

done_testing();

[download]

In reply to Re: Fuzzy How-Many by repellent
in thread Fuzzy How-Many by Xiong

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