In the examples above I'm looking to see the base point movements of +1,+4 and -5.

I'm not really sure I understand, but maybe something like this?

#!/usr/bin/perl

use strict;
use warnings;

print base_point_movement($_),"\n" for (
          1.5553 - 1.5552,
          0.9984 - 0.998,
          100.25 - 100.3,
      );

sub base_point_movement {
    my $diff = shift;
    (sprintf "%+e", $diff) =~ /([+-]\d)\./;
    return $1;
}
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Output:

+1
+4
-5
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Update: as has been pointed out, this only works for +/-9 — which is not too surprising, as it just extracts the before-comma digit of the difference in exponential floating-point representation  (not knowing what a "base point" is defined as, I figured it might suffice...).

Anyhow, here's another variant, kind of extending the idea to what might have been meant — though honestly, I don't have the foggiest whether that's what the OP had in mind.  (Of course, this will also run into problems when the number of significant digits exceeds floating-point precision.)

sub base_point_movement {
    my $bpm = sprintf "%+e", shift;
    $bpm =~ s/0*e.*$//;
    $bpm =~ tr/.//d;
    return $bpm;
}

print base_point_movement($_),"\n" for (
          1.5553    - 1.5552,      # +1
          1.5553    - 1.55,        # +53
          1.56      - 1.55,        # +1
          155.53    - 155.52,      # +1
          15553     - 15552,       # +1
          1555300   - 1555200,     # +1
          1555300   - 155520,      # +139978
          1555.300  - 1555.20,     # +1
          1555300   - 1555201,     # +99
          1.5553    - 1.555201,    # +99
          1.5553    - 1.55521,     # +9
          1.5553    - 1.555301,    # -1
          1.5553    - 1.55530101,  # -101
          1.5553    - 1.5553101,   # -101
          15553     - 15552.9999,  # +1
          15553.001 - 15552.999,   # +2
          15553.001 - 15552.9999,  # +11
          15553.1   - 15552.99,    # +11
          # etc. (you get the idea :)
      );
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