#! perl -slw
use strict;
use List::Util qw[ reduce ];
use Data::Dumper;

my @AoH;
while( <DATA> ) {
    chomp;
    push @AoH,  reduce{ 
        my $r={}; $r->{$b}=$a; $r;
    } reverse split /:|=/, $_;
}

print Dumper \@AoH;

__DATA__
a1:a2:a3=key1
a1:a2=key2
b1:b2:b3:b4:b5:b6:b7:b8:b9=key3