my @sects= qw( 1 2 2.2 2.13 2.1.7 3.4a 10.1 10.10 10.1a 1a.2 );
my %sects;
for ( @sects ) {
    ( my $key = $_ ) =~ s/(\d+)/substr("0000$1",-4)/ge;
    $sects{$key} = $_;
}
print "@sects{sort keys %sects}\n";