assuming everything's already in the right order, just keep assigning the latest value to the key and at the end you'll have all your values in a hash. this is the quick and dirty way which assumes you don't have 100Ks to Ms of keys. if so, write it a little more C-like.

#!perl -wl

use strict;

my %h;
$h{substr($_, 0, 1)} = substr($_, 2, 1) while <DATA>;
print "$_.$h{$_}" for sort keys %h;

__DATA__
1.1
1.2
1.3
2.0
2.1
2.2
3.4
4.5
4.6
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