The OP's rounding is inconsistent. It rounds by 1) ten, 2) ten, 3) ten, 4) hundred for numbere of magnitude 1) unit, 2) ten, 3) hundred, 4) thousand. There's no pattern there.

So, I understood the question as "round up to one significant digit, be it 10s, 100s, 1000s, ... ". It even handles negative numbers, and numbers between +-1 and 0!

sub nearest {
   local $_ = 0+$_[0];
   return 0 unless $_;

   my $f = $_ <=> 0;
   $_ = abs($_);

   while ($_ >= 10) { $_ /= 10; $f *= 10; }
   while ($_ <   1) { $_ *= 10; $f /= 10; }

   return int($_ + 0.5) * $f;
}


sub up_nearest {
   local $_ = 0+$_[0];
   return 0 unless $_;

   my $f = $_ <=> 0;
   $_ = abs($_);

   while ($_ >= 10) { $_ /= 10; $f *= 10; }
   while ($_ <   1) { $_ *= 10; $f /= 10; }

   return (int($_) == $_ ? $_ : (int($_) + 1)) * $f;
}


foreach (
  0,
  5,
  10,
  50,
  0.1,

  5.1,
  11,
  51,
  0.11,

  5.5,
  15,
  55,
  0.15,

  5.9,
  19,
  59,
  0.19,
) {
   printf("%12.6f: %4s\n",  $_,    nearest( $_));
   printf("%12.6f| %4s\n",  $_, up_nearest( $_));
   printf("%12.6f: %4s\n", -$_,    nearest(-$_)) if $_;
   printf("%12.6f| %4s\n", -$_, up_nearest(-$_)) if $_;
}
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