Binary floating point cannot represent 0.01 from base 10 exactly, close but not exact. Here is an example of rounding the printout using printf():

Update: I got a suggestion to make my warning about the use of $a and $b more prominent. $a and $b are Perl reserved variables and are used by sort amoungst other functions. Users should not use these variables. Use $x or $y or something else instead. Following my own advice, I changed from $a to $x below. Here the OP's use of $a "worked", but it is a bad habit to get into.

#!usr/bin/perl
use strict;
use warnings;

print "====float =======\n";

my $x=-50.123456;
for my $i (0...19)
{
  printf "%i\t%.3f\n", $i,$x;
   $x+=0.01;
}

$x=-71.123456;
for my $i (0...19)
{
   printf "%i\t%.3f\n", $i,$x;
   $x+=0.01;
}
__END__
====float =======
0    -50.123
1    -50.113
2    -50.103
3    -50.093
4    -50.083
5    -50.073
6    -50.063
7    -50.053
8    -50.043
9    -50.033
10    -50.023
11    -50.013
12    -50.003
13    -49.993
14    -49.983
15    -49.973
16    -49.963
17    -49.953
18    -49.943
19    -49.933
0    -71.123
1    -71.113
2    -71.103
3    -71.093
4    -71.083
5    -71.073
6    -71.063
7    -71.053
8    -71.043
9    -71.033
10    -71.023
11    -71.013
12    -71.003
13    -70.993
14    -70.983
15    -70.973
16    -70.963
17    -70.953
18    -70.943
19    -70.933
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Update: added similar comment above and changed $a to $x.
Here it is ok, but $a and $b are reserved Perl variables and shouldn't be used by users. Use $x or $y instead.