use Data::Dumper;
print Dumper([build_dictionnary()]);

sub build_dictionnary{
    my $line="ABCDEF";
    my @dic;
    while(length($line)>2){
        $line =~ s/([A-Z]([A-Z]{2}))/$2/;
        push(@dic, $1);
    }
    return @dic;
}

# Output (whitespace compressed):
# $VAR1 = [ 'ABC', 'BCD', 'CDE', 'DEF' ];